[Sema] Use MSVC inner class behavior on Itanium

Windows Itanium aims to use MSVC export and import semantics. Inner
class members shouldn't be exported on a dllexport explicit
instantiation definition of the outer class, and they shouldn't be
imported on a dllimport explicit instantiation declaration of the outer
class (instead a local copy should be emitted). We were doing the first
but not the second, and this mismatch can lead to link errors. Fix the
behavior and add tests for both.

Differential Revision: https://reviews.llvm.org/D32213

llvm-svn: 300804

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