If the expression parser is unable to complete a TagDecl

in the context in which it was originally found, the
expression parser now goes hunting for it in all modules
(in the appropriate namespace, if applicable).  This means
that forward-declared types that exist in another shared
library will now be resolved correctly.

Added a test case to cover this.  The test case also tests
"frame variable," which does not have this functionality
yet.

llvm-svn: 146204