[libcxx][test] compiler options are non-portable

... it's easier to suppress warnings internally, where we can detect the compiler.

* Rename `TEST_COMPILER_C1XX` to `TEST_COMPILER_MSVC`
* Rename all `TEST_WORKAROUND_C1XX_<meow>` to `TEST_WORKAROUND_MSVC_<meow>`

Differential Revision: https://reviews.llvm.org/D117422