Allow TLS vars in dllimport/export functions; only inline dllimport functions when safe (PR24593)

This patch does two things:

1) Don't error about dllimport/export on thread-local static local variables.
   We put those attributes on static locals in dllimport/export functions
   implicitly in case the function gets inlined. Now, for TLS variables this
   is a problem because we can't import such variables, but it's a benign
   problem becase:

2) Make sure we never inline a dllimport function TLS static locals. In fact,
   never inline a dllimport function that references a non-imported function
   or variable (because these are not defined in the importing library). This
   seems to match MSVC's behaviour.

Differential Revision: http://reviews.llvm.org/D12422

llvm-svn: 246338