build: fix clang-cl builds, add CI job

- appveyor: add build-only job for clang-cl.

- cmake: `-pedantic-errors` enables `-Werror,-Wlanguage-extension-token`
  automatically, which makes `__int64` detection fail.
  Explictly disable this compiler warning for clang-cl to make the
  feature detection work and to accept `__int64` in the source code.

- cmake: disable `-Wlanguage-extension-token` warning for clang-cl
  to fix these when encountering `__int64`:
  ```
  lib/formdata.c(797,29): error : extension used [-Werror,-Wlanguage-extension-token]
  lib/warnless.c(117,33): error : extension used [-Werror,-Wlanguage-extension-token]
  lib/warnless.c(60,28): message : expanded from macro 'CURL_MASK_SCOFFT'
  lib/warnless.c(59,38): message : expanded from macro 'CURL_MASK_UCOFFT'
  include\curl/system.h(352,40): message : expanded from macro 'CURL_TYPEOF_CURL_OFF_T'
  ```

- make `__GNUC__` warning suppressions apply to `__clang__` too.
  Necessary for clang-cl, which defines the latter, but not the former.
  (Regular clang defines both.)

- examples: fix clang-cl compiler warning in `http2-upload.c`.
  ```
  docs\examples\http2-upload.c(56,5): error : no previous prototype for function 'my_gettimeofday' [-Werror,-Wmissing-prototypes]
  docs\examples\http2-upload.c(56,1): message : declare 'static' if the function is not intended to be used outside of this translation unit
  ```

- unit2604: add missing `#pragma GCC diagnostic pop`.
  Follow-up to e53523fef0 #14859

- unit1652: limit compiler warning suppression to GCC.
  They do not affect clang builds.
  Follow-up to 71cf0d1fca #14772

Closes #15449

tests/libtest/lib557.c

@@ -37,7 +37,7 @@
 
 #include "memdebug.h"
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wformat"
 #pragma GCC diagnostic ignored "-Wformat-extra-args"
@@ -1559,6 +1559,6 @@ CURLcode test(char *URL)
     return CURLE_OK;
 }
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic pop
 #endif

tests/unit/unit1398.c

@@ -23,7 +23,7 @@
  ***************************************************************************/
 #include "curlcheck.h"
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wformat"
 #endif
@@ -186,6 +186,6 @@ fail_unless(rc == 128, "return code should be 128");
 
 UNITTEST_STOP
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic pop
 #endif

tests/unit/unit1652.c

@@ -26,15 +26,6 @@
 #include "urldata.h"
 #include "sendf.h"
 
-#ifdef __GNUC__
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wformat"
-#pragma GCC diagnostic ignored "-Wformat-zero-length"
-#if !defined(__clang__) && __GNUC__ >= 7
-#pragma GCC diagnostic ignored "-Wformat-overflow"
-#endif
-#endif
-
 /*
  * This test hardcodes the knowledge of the buffer size which is internal to
  * Curl_infof(). If that buffer is changed in size, this tests needs to be
@@ -101,6 +92,15 @@ static int verify(const char *info, const char *two)
 
 UNITTEST_START
 
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat"
+#pragma GCC diagnostic ignored "-Wformat-zero-length"
+#if __GNUC__ >= 7
+#pragma GCC diagnostic ignored "-Wformat-overflow"
+#endif
+#endif
+
 /* Injecting a simple short string via a format */
 msnprintf(input, sizeof(input), "Simple Test");
 Curl_infof(testdata, "%s", input);
@@ -146,8 +146,8 @@ Curl_infof(testdata, "%s", input);
 fail_unless(strlen(output) == 2051, "Truncation of infof input 3");
 fail_unless(output[sizeof(output) - 1] == '\0', "Truncation of infof input 3");
 
-UNITTEST_STOP
-
-#ifdef __GNUC__
+#if defined(__GNUC__) && !defined(__clang__)
 #pragma GCC diagnostic pop
 #endif
+
+UNITTEST_STOP

tests/unit/unit2604.c

@@ -46,7 +46,7 @@ struct set {
 UNITTEST_START
 #ifdef USE_SSH
 {
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Woverlength-strings"
 #endif
@@ -78,7 +78,7 @@ UNITTEST_START
     { NULL, NULL, NULL, NULL, CURLE_OK }
   };
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic warning "-Woverlength-strings"
 #endif
 
@@ -114,6 +114,10 @@ UNITTEST_START
 
   free((void *)list[0].cp);
 }
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
 #endif
 
 UNITTEST_STOP

tests/unit/unit3200.c

@@ -47,7 +47,7 @@ static CURLcode unit_stop(void)
   return CURLE_OK;
 }
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Woverlength-strings"
 #endif
@@ -78,7 +78,7 @@ static const char *filecontents[] = {
   "LINE1\x1aTEST"
 };
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic warning "-Woverlength-strings"
 #endif
 
@@ -173,7 +173,7 @@ UNITTEST_START
   return (CURLcode)rc;
 UNITTEST_STOP
 
-#ifdef __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic pop
 #endif