[clang][DebugInfo] Allow function-local type-like entities to be scoped within a lexical block (5/5)

[krisb]

Summary

This is a part from D113743 split to make it easier to test and review.

It also enables lexical blocks as a scope for types and type-like entities
only if -debugger-tunning=gdb. lldb can't (yet) handle lexical block scoped
types (see D115277 for details), other debuggers were not verified (yet).

[aprantl]

Sgtm.

[dblaikie]

clang/lib/CodeGen/CGDebugInfo.cpp

231–234

Rather than doing an extra lookup in the assert, could probably check the return value of insert instead and assert based on that?

[krisb]

Applied review comments.