allow it to complete types on behalf of any AST context
(including the "scratch" AST context associated with
the target), I scrapped its role as intermediary between
the Clang parser and ClangExpressionDeclMap, and instead
made ClangExpressionDeclMap inherit from ClangASTSource.

After this, I will migrate the functions that complete
types and perform namespace lookups from
ClangExpressionDeclMap to ClangASTSource.  Ultimately
ClangExpressionDeclMap's only responsiblity will be to
look up variables and ensure that they are materialized
and dematerialized correctly.

llvm-svn: 143253