[DwarfDebug] Support emitting function-local declaration for a lexical block

This is another attempt to make function-local declarations
(like static variables, structs/classes and other) be correctly
emitted within a lexical (bracketed) block.

Fixes https://bugs.llvm.org/show_bug.cgi?id=19238.

Differential Revision: https://reviews.llvm.org/D113741