AST: Address of dllimport variables isn't constant

The address of dllimport variables isn't something that can be
meaningfully used in a constexpr context and isn't suitable for
evaluation at load-time.  They require loads from memory to properly
evaluate.

This fixes PR19955.

Differential Revision: http://reviews.llvm.org/D4250

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211568 91177308-0d34-0410-b5e6-96231b3b80d8
AST: Address of dllimport functions isn't constant

The address of dllimport functions can be accessed one of two ways:
- Through the IAT which is symbolically referred to with a symbol
  starting with __imp_.
- Via the wrapper-function which ends up calling through the __imp_
  symbol.

The problem with using the wrapper-function is that it's address will
not compare as equal in all translation units.  Specifically, it will
compare unequally with the translation unit which defines the function.

This fixes PR19955.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211570 91177308-0d34-0410-b5e6-96231b3b80d8