For code completion in C++ member access expressions and tag names,

look into the current scope for anything that could start a
nested-names-specifier. These results are ranked worse than any of the
results actually found in the lexical scope.

Perform a little more pruning of the result set, eliminating
constructors, __va_list_tag, and any duplication of declarations in
the result set. For the latter, implemented
NamespaceDecl::getCanonicalDecl.

llvm-svn: 82231