Don't warn about selectany on implicitly inline variables
This avoids a -Wignored-attribute warning on the code pattern Microsoft
recommends for integral const static data members defined in headers
The attribute is redundant, but it is necessary when compiling in C++14
modes with /Za, which disables MSVC's extension that treats such
variables as implicitly inline.
Reviewers: epastor, thakis, hans
Differential Revision: https://reviews.llvm.org/D67426