[MinGW] Move code for indicating "potentially not DSO local" into shouldAssumeDSOLocal. NFC.

On Windows, if shouldAssumeDSOLocal returns false, it's either a
dllimport reference, or a reference that we should treat as non-local
and create a stub for.

Clean up AArch64Subtarget::ClassifyGlobalReference a little while
touching the flag handling relating to dllimport.

Differential Revision: https://reviews.llvm.org/D51590

llvm-svn: 341402