clang-cl: With -fmsc-version=1900, use MSVS2015 diag formatting.

Remove tests that have neither a triple nor an explicit -fmsc-version flag,
since in the absence of an -fmsc-version flag, the implicit value of the flag
is 17 (MSVC2013) with MSVC triples but 0 (not set) for other triples, and
the default triple is platform dependent.

This relands r263974 with a test fix.

llvm-svn: 264210