[modules] Before checking whether the controlling macro of a header is defined,

update the identifier in case we've imported a definition of the macro (and
thus the contents of the header) from a module.

Also fold ExternalIdentifierLookup into ExternalPreprocessorSource; it no longer
makes sense to keep these separate now that the only user of the former also
needs the latter.

llvm-svn: 241137

clang/lib/Basic/IdentifierTable.cpp

@@ -71,8 +71,6 @@ IdentifierIterator *IdentifierInfoLookup::getIdentifiers() {
   return new EmptyLookupIterator();
 }
 
-ExternalIdentifierLookup::~ExternalIdentifierLookup() {}
-
 IdentifierTable::IdentifierTable(const LangOptions &LangOpts,
                                  IdentifierInfoLookup* externalLookup)
   : HashTable(8192), // Start with space for 8K identifiers.