Move some attribute diagnostic helper functions; NFC.

These functions were local to SemaDeclAttr.cpp, but these functions are
useful in general (for instance, for statement or type attribute
processing). This refactoring is in advance of beginning to tablegen
diagnostic checks for statement attributes the way we already do for
declaration attributes.

There is one functional change in here as a drive-by. The
external_source_symbol attribute had one of its diagnostic checks
inside of an assert, which was corrected.