Don't run Clang in MS compatibility mode on Windows (-fno-ms-compatibility).

Fixes #762. Thanks to d0k in #llvm for the helpful insight!

Finally found the cause to the mysterious issues on Windows. It turns out that Clang enables Microsoft-friendly emulation mode by default, and that's the reason why it fails to parse libcxx properly. This commit removes that, and fixes the Clang 3.2 libcxx problem.

(There are still other build errors found after that, reported in #779)