Represent an APValue based on a Decl as that Decl, rather than a DeclRefExpr

Represent an APValue based on a Decl as that Decl, rather than a DeclRefExpr
or MemberExpr which refers to it. As a side-effect, MemberExprs which refer to
static member functions and static data members are now emitted as constant
expressions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144468 91177308-0d34-0410-b5e6-96231b3b80d8