[Error] Add source location to cantFail

[hintonda]

Summary

Add llvm_cantFail macro, a drop in replacement for llvm::cantFail,
that adds source location, similar to assert, for !NDEBUG builds.

Refactor to extract error handling code and place it and new annotated
versions of cantFail that are called by the new macro in a detail
namespace, and change all instances in llvm to use the new macro.

[hintonda]

• Replace macro magic with matching 3-parameter template functions.

[lhames]

Hi Don,

This is a great idea!

I think it would actually be reasonable to have 'cantFail' be the macro though, and move the existing functions (which become an implementation detail) into a detail namespace: We want cantFail to be succinct as part of its value is clarifying flow when we know (from the surrounding context) that errors will not be generated.

[hintonda]

• Replace macro magic with matching 3-parameter template functions.
• Refactor cantFail move into detail namespace. Change macro name to

[hintonda]

• Add back original llvm::cantFail signatures so they'll still be

[hintonda]

Append "_annotated" suffix to detail functions.
Add "llvm_" prefix to macros.
Revert changes to all non-llvm project files.
Update calls in llvm to use new macro.

[hintonda]

@lhames, I think I've applied all your verbal changes, but please let me
know if I missed anything.

Essentially, previous behavior is maintained, but users can opt-in to the
new behavior, adding file and line numbers, by switching to the new macro.
I also updated all of llvm, but excluded other projects for the time being.
Most of them use llvm::cantFail, and could be updated as needed via a
script, e.g., something like this would do it:

find clang -type f \( -name "*.cpp" -o -name "*.h" \) -exec grep -l "cantFail" \{\} \; | xargs sed -i.bak 's/llvm::cantFail/llvm_cantFail/'