[CMake][MSVC] Compile with `/permissive-`

This turns off a bunch of non-standard behaviors in MSVC. LLVM, as a
portable codebase, should build correctly without those behaviors. Note
that `/permissive-` implies `/Zc:strictStrings` and `/Zc:rvalueCast`.

See also:
https://docs.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance

Differential Revision: https://reviews.llvm.org/D125263