acquiring the current TU's clang::IdentifierTable

is there a way to acquire the IdentifierTable for the current TU to
search through it for "interesting" entries? any other method to
achieve the same goal is fine.
I need to do some search for re-used identifiers, outer-scope
identifiers hiding the ones with the same name, whether a tag is a
unique identifier... checks along those lines. I could just match and
store all the identifiers in a TU and then run the checks in
runOnEndOfTranslationUnit but that would require extra resources.
it would be more convenient if i could get that in an
ASTFrontendAction. is that possible?