[ProfileData] PR33517: Check for failure of symtab creation

With PR33517, it became apparent that symbol table creation can fail
when presented with malformed inputs. This patch makes that sort of
error detectable, so llvm-cov etc. can fail more gracefully.

Specifically, we now check that function names within the symbol table
aren't empty.

Testing: check-{llvm,clang,profile}, some unit test updates.
llvm-svn: 305765