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

appveyor.yml

@@ -89,6 +89,16 @@ environment:
       SCHANNEL: 'ON'
       ENABLE_UNICODE: 'OFF'
       EXAMPLES: 'ON'
+    - job_name: 'CMake, VS2022, Debug, x64, Schannel, Static, Unicode, Build-tests & examples, clang-cl'
+      APPVEYOR_BUILD_WORKER_IMAGE: 'Visual Studio 2022'
+      BUILD_SYSTEM: CMake
+      PRJ_GEN: 'Visual Studio 17 2022'
+      TARGET: '-A x64'
+      PRJ_CFG: Debug
+      SCHANNEL: 'ON'
+      ENABLE_UNICODE: 'ON'
+      EXAMPLES: 'ON'
+      TOOLSET: 'ClangCl'
     - job_name: 'CMake, VS2022, Debug, x64, Schannel, Static, Unicode, Build-tests'
       APPVEYOR_BUILD_WORKER_IMAGE: 'Visual Studio 2022'
       BUILD_SYSTEM: CMake