Introduce the feature "can-remove-opened-file" to suppress tests. [PR18856]
check-clang: Introduce the feature "dw2" to suppress CodeGenCXX/debug-info-namespace.cpp with *-win32.

FIXME: Could we add itanium triple here?
check-clang: Introduce the feature "non-ms-sdk" to suppress Driver/nostdincxx.cpp on msvc.
clang/unittests/AST,ASTMatchers: Remove _MSC_VER.