[runtimes] Don't use -Wall on clang-cl builds

[philnik]

Summary

-Wall on clang-cl is equivalent to -Weverything on clang. We already add the correct warning flag depending whether we are in an MSVC-like environment, so just remove it from the list of flags that get passed unconditionally.

[philnik]

Other fix

runtimes/cmake/Modules/WarningFlags.cmake

 include(HandleFlags)
 
 # Warning flags ===============================================================
 function(cxx_add_warning_flags target enable_werror enable_pedantic)
   target_compile_definitions(${target} PUBLIC -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
   if (MSVC)
     # -W4 is the cl.exe/clang-cl equivalent of -Wall. (In cl.exe and clang-cl,
     # -Wall is equivalent to -Weverything in GCC style compiler drivers.)
     target_add_compile_flags_if_supported(${target} PRIVATE -W4)
   else()
     target_add_compile_flags_if_supported(${target} PRIVATE -Wall)
   endif()
   # TODO: Should -Wconversion be enabled?
   target_add_compile_flags_if_supported(${target} PRIVATE
-      -Wall
       -Wextra
       -Wnewline-eof
       -Wshadow
       -Wwrite-strings
       -Wno-unused-parameter
       -Wno-long-long
       -Werror=return-type
       -Wextra-semi