clang-cl: Emit narrowing diag for initializer lists if -fmsc-version is at...

clang-cl: Emit narrowing diag for initializer lists if -fmsc-version is at least 1900 (i.e. MSVC2015).

Diagnostics for narrowing conversions in initializer lists are currently
DefaultIgnored in Microsoft mode. But MSVC 2015 did add warnings about
narrowing conversions (C2397), so clang-cl can remove its special case code if
MSCompatibilityVersion is new enough.

(In MSVC, C2397 is just a warning and in clang it's default-mapped to an error,
but it can be remapped, and disabled with -Wno-c++11-narrowing, so that should
be fine.)

Fixes PR37314.
https://reviews.llvm.org/D48296

llvm-svn: 335082