[DebugInfo] Do not replace existing nodes from DICompileUnit

When creating a new DIBuilder with an existing DICompileUnit, load the
DINodes from the current DICompileUnit so they don't get overwritten.
This is done in the MachineOutliner pass, but it didn't change the CU so
the bug never appeared. We need this if we ever want to add DINodes to
the CU after it has been created, e.g., DIGlobalVariables.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D114556