[MSCV][dllexport/dllimport][PS] Allow UniqueExternal linkage classes with dllexport...

[MSCV][dllexport/dllimport][PS] Allow UniqueExternal linkage classes with dllexport/dllimport

MSVC allows instantiations of exported or imported template classes with template
parameters that have internal linkage. Clang now allows it in Microsoft mode and for
the Playstation platform. This partially addresses issue 56068.

Note that MSVC also allows explicit dllexport/dllimport attributes on classes
with internal linkage (e.g. local classes or classes declared in anonymous name spaces).
Clang continues to reject such declarations.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D146338