[libc++] Respect Windows Store app CRT restrictions

Some CRT APIs are unavailable for Windows Store apps [1]. Detect when
we're targeting the Windows Store and don't try to refer to non-existent
CRT functions in that case. (This would otherwise lead to a compile
error when using the libc++ headers and compiling for Windows Store.)

[1] https://docs.microsoft.com/en-us/cpp/cppcx/crt-functions-not-supported-in-universal-windows-platform-apps

Differential Revision: https://reviews.llvm.org/D31737

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@299625 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed