Extract an isReservedName() function

We'll want to share the implementation if anything else decides to check
for reserved names in future, so make this little snippet of code more
discoverable.

Also remove the __va_list_tag and __builtin_va_list special-case
checks. They're leftovers from before when the reserved name logic was
added.

No change in functionality.
CodeGenAction::ExecuteAction(): check for invalid LLVM source locations

Add sign checks to deal with the fact that IR parser line/column pairs are
signed integers and sometimes invalid.

The crash path is potentially triggered by corrupt '.bc' files in practice,
though I don't have a binary input test case that can be checked-in right now.

(Unfortunately the backend itself crashes on various ill-formed '.bc' inputs so
this bandage isn't as helpful as it appears yet.)
Remove some incorrect test suppressions

These don't actually require any registered backend to run.

This commit tests the water with a handful of fixes for what is a more
widespread problem.