[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