[DebugInfo][CodeView] Change in line tables only mode to emit type information

for function scopes, rather than using the qualified name.

In line-tables-only mode, we used to emit qualified names as the display name for functions when using CodeView.
This patch changes to emitting the parent scopes instead, with forward declarations for class types.
The total object file size ends up being slightly smaller than if we use the full qualified names.

Differential Revision: https://reviews.llvm.org/D94639