Add using directives to the clang::DeclContext and fix decls for variables inside namespaces

Summary: Supports the parsing of the "using namespace XXX" and "using XXX::XXX" directives. Added ambiguity errors when it two decls with the same name are encountered (see comments in TestCppNsImport). Fixes using directives being duplicated for anonymous namespaces. Fixes GetDeclForUID for specification DIEs.

Reviewers: sivachandra, chaoren, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12897

llvm-svn: 247836