[libc++][modules] Guard #error against MSVC headers on Windows

[mordante]

The libc++ test suite sees the MSVC STL headers and may conclude these are libc++ headers when inspecting the name. Modules guard against forgetting to export new headers, and this gets false positives if it finds MSVC STL headers.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Mark de Wever (mordante)

The libc++ test suite sees the MSVC STL headers and may conclude these are libc++ headers when inspecting the name. Modules guard against forgetting to export new headers, and this gets false positives if it finds MSVC STL headers.