[libc++] Use functions instead of global variables to set libc++ build flags

This commit follows the trend of doing things per-target instead of
modifying the C++ flags globally. It does so for visibility-related
flags, other basic build flags and Windows-specific flags.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373517 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed