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.sh

@@ -38,6 +38,7 @@ openssl_root="$(cygpath "${openssl_root_win}")"
 if [ "${BUILD_SYSTEM}" = 'CMake' ]; then
   options=''
   [[ "${TARGET:-}" = *'ARM64'* ]] && SKIP_RUN='ARM64 architecture'
+  [ -n "${TOOLSET:-}" ] && options+=" -T ${TOOLSET}"
   [ "${OPENSSL}" = 'ON' ] && options+=" -DOPENSSL_ROOT_DIR=${openssl_root_win}"
   [ -n "${CURLDEBUG:-}" ] && options+=" -DENABLE_CURLDEBUG=${CURLDEBUG}"
   [ "${PRJ_CFG}" = 'Debug' ] && options+=' -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG='
@@ -62,6 +63,9 @@ if [ "${BUILD_SYSTEM}" = 'CMake' ]; then
     '-DCMAKE_INSTALL_PREFIX=C:/curl' \
     "-DCMAKE_BUILD_TYPE=${PRJ_CFG}" \
     '-DCURL_USE_LIBPSL=OFF'
+  if false; then
+    cat _bld/CMakeFiles/CMakeConfigureLog.yaml 2>/dev/null || true
+  fi
   echo 'curl_config.h'; grep -F '#define' _bld/lib/curl_config.h | sort || true
   # shellcheck disable=SC2086
   if ! cmake --build _bld --config "${PRJ_CFG}" --parallel 2 -- ${BUILD_OPT:-}; then
@@ -119,10 +123,6 @@ else
   echo "Skip running curl.exe. Reason: ${SKIP_RUN}"
 fi
 
-if false; then
-  cat CMakeFiles/CMakeConfigureLog.yaml 2>/dev/null || true
-fi
-
 # build tests
 
 if [[ "${TFLAGS}" != 'skipall' ]] && \