[DebugInfo][CallSites] Remove decl subprograms from 'retainedTypes:'
After the D70350, the retainedTypes: isn't being used for the purpose
of call site debug info for extern calls, so it is safe to delete it
from IR representation.
We are also adding a test to ensure the subprogram isn't stored within
the retainedTypes: from corresponding DICompileUnit.
Differential Revision: https://reviews.llvm.org/D80369