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

CMake/PickyWarnings.cmake

@@ -31,6 +31,9 @@ if(CURL_WERROR AND
      NOT CMAKE_VERSION VERSION_LESS 3.23.0) OR  # to avoid check_symbol_exists() conflicting with GCC -pedantic-errors
    CMAKE_C_COMPILER_ID MATCHES "Clang"))
   list(APPEND _picky "-pedantic-errors")
+  if(MSVC)  # clang-cl
+    list(APPEND _picky "-Wno-language-extension-token")  # Override default error to make __int64 size detection pass
+  endif()
 endif()
 
 if(APPLE AND
@@ -130,9 +133,13 @@ if(PICKY_COMPILER)
         ${_picky_common_old}
         -Wshift-sign-overflow              # clang  2.9
         -Wshorten-64-to-32                 # clang  1.0
-        -Wlanguage-extension-token         # clang  3.0
         -Wformat=2                         # clang  3.0  gcc  4.8
       )
+      if(NOT MSVC)
+        list(APPEND _picky_enable
+          -Wlanguage-extension-token         # clang  3.0  # Avoid for clang-cl to allow __int64
+        )
+      endif()
       # Enable based on compiler version
       if((CMAKE_C_COMPILER_ID STREQUAL "Clang"      AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.6) OR
          (CMAKE_C_COMPILER_ID STREQUAL "AppleClang" AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 6.3))