[DebugInfo] Fix debug-info generation for block invocations so that we set the LinkageName

Currently when generating debug-info for a BlockDecl we are setting the Name to the mangled name and not setting the LinkageName.
This means we see the mangled name for block invcations ends up in DW_AT_Name and not in DW_AT_linkage_name.

This patch fixes this case so that we also set the LinkageName as well.

Differential Revision: https://reviews.llvm.org/D73282