Properly print unnamed TagDecl objects in diagnostics

The diagnostics engine is very smart about being passed a NamedDecl to
print as part of a diagnostic; it gets the "right" form of the name,
quotes it properly, etc. However, the result of using an unnamed tag
declaration was to print '' instead of anything useful.

This patch causes us to print the same information we'd have gotten if
we had printed the type of the declaration rather than the name of it,
as that's the most relevant information we can display.

Differential Revision: https://reviews.llvm.org/D134813