[libcxx] Don't add -Wall when building in MSVC mode

The MSVC -Wall (or /Wall) option maps (in clang-cl) to the GCC style
option -Weverything, which we don't really want. Instead use -W4 which
is the corresponding MSVC option.

This silences the build with clang-cl, which previously used to
output 100 warnings per translation unit.

Differential Revision: https://reviews.llvm.org/D98035