Mainly, changes the default from -1 to 0 in DiagnosticLoc, still trying
to keep reusing that. Nothing except for the lowered output is affected,
so I think this is fine.
Also, have lowering consistently call GetDiagnosticLoc.
Pulls in one of the CHECKs suggested from #4251
Co-authored-by: David Blaikie <dblaikie@gmail.com>
I'm trying to make the line of code more clearly nested in crash output
(the way it is, I sometimes forget about it). Also,
`Check::HandleFunctionDecl` is the old naming scheme, it's now all
`Check::HandleParseNode`, so I'm replacing that.
Before:
```
3. extern_library_owner.carbon:6:1: Check::HandleFunctionDecl
extern fn F();
^~~~~~~~~~~~~~
#0 0x0000564c0057ef1d llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) ...
```
After:
```
3. extern_library_owner.carbon:6:1: checking FunctionDecl
extern fn F();
^~~~~~~~~~~~~~
#0 0x00005629029ffd9d llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) ...
```
When we crash, include the source location of the parse node that we
were handling, as well as the name of the check function that we were
calling. Also include the source snippet, since it's easy to do so, and
may avoid the need to look at the input file in some cases.
josh11bandJosh L
Jon Ross-PerkinsandDavid Blaikie
Richard Smith