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

`-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.

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D146378

runtimes/cmake/Modules/WarningFlags.cmake

@@ -12,7 +12,6 @@ function(cxx_add_warning_flags target enable_werror enable_pedantic)
   endif()
   # TODO: Should -Wconversion be enabled?
   target_add_compile_flags_if_supported(${target} PRIVATE
-      -Wall
       -Wextra
       -Wnewline-eof
       -Wshadow