Don't warn about selectany on implicitly inline variables

Summary:
This avoids a -Wignored-attribute warning on the code pattern Microsoft
recommends for integral const static data members defined in headers
here:
https://docs.microsoft.com/en-us/cpp/build/reference/microsoft-extensions-to-c-and-cpp?view=vs-2019

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.

Fixes PR43270

Reviewers: epastor, thakis, hans

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67426

llvm-svn: 371749