Adjust check's node formatting in crash output.

[jonmeow]

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) ...

[chandlerc]

LG