Improve location information in the representation of namespace

aliases. Previously, the location of the alias was at the "namespace"
keyword. Now, it's on the identifier being declared (as is the custom
for Clang), and we keep a separate source location for the "namespace"
keyword.

Also, added a getSourceRange() member function to NamespaceAliasDecl
to correctly compute the source range.

Finally, removed a bunch of setters from NamespaceAliasDecl and gave
ASTReaderDecl friendship so that it could set the corresponding fields
directly.

llvm-svn: 112681