Sema: add a helper for enumerating the TST id (NFC)

The same pattern was repeated a few times.  Create a trivial helper method to
map the Type Specifier to an ID for the diagnostic.  Flip the selection order on
one of the diagnostic messages to get the same ordering across all of the
messages.  This makes the emission of the diagnostic slightly more legible by
changing the cascading ternary into a switch in a function.  NFC.

llvm-svn: 240251