[MIRParser][nfc] Factor out code parsing debug MD nodes

This commit splits a function that both parses MD nodes from YAML into
DI{Expr,Loc,Variable} objects AND adds an entry to the MF variable table, so
that each of those jobs is done separately.

It will enable subsequent patches to reuse the MD node parsing code.

Differential Revision: https://reviews.llvm.org/D149870