[libc++] Account for Microsoft CRT const overloads
[libc++] Account for Microsoft CRT const overloads

Microsoft's CRT already provides the const overloads, and it defines the
`_CRT_CONST_CORRECT_OVERLOADS` macro to indicate their presence. Check
for this macro before attempting to define our own const-correct
overloads, to avoid compiler warnings about casts dropping const
qualifiers.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@313377 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx