[MS ABI] Explicit specialization of static data members are weak
Normally, explicit specializations are treated like strong external
definitions. However, MSVC treats explicit specializations of static
data members as weak. MSVC 2013's <regex> implementation has such an
explicit specialization which leads to clang emitting a strong
definition in each translation unit which includes it. Tweak clang's
linkage calculation to give such entities GVA_StrongODR linkage instead.
This fixes PR24165.
llvm-svn: 242592