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

lib/content_encoding.c

@@ -33,13 +33,13 @@
 #endif
 
 #ifdef HAVE_BROTLI
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 /* Ignore -Wvla warnings in brotli headers */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wvla"
 #endif
 #include <brotli/decode.h>
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic pop
 #endif
 #endif