[Coverage] PR33517: Check for failure to load func records

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 records loaded from corrupted coverage
mapping data are rejected, e.g when the recorded function name is garbage.

Testing: check-{llvm,clang,profile}, some unit test updates.
llvm-svn: 305767