[llvm][DebugInfo][clang] Finalize all declaration subprograms in DIBuilder::finalize() (#139914)

DIBuilder began tracking definition subprograms and finalizing them in
`DIBuilder::finalize()` in eb1bb4e419.
Currently, `finalizeSubprogram()` attaches local variables, imported
entities, and labels to the `retainedNodes:` field of a corresponding
subprogram.

After 75819aedf, the definition and some declaration subprograms are
finalized in `DIBuilder::finalize()`:
`AllSubprograms` holds references to definition subprograms.
`AllRetainTypes` holds references to declaration subprograms.
For DISubprogram elements of both variables, `finalizeSubprogram()` was
called there.

However, `retainTypes()` is not necessarily called for every declaration
subprogram (as in 40a3fcb0).

DIBuilder clients may also want to attach DILocalVariables to
declaration subprograms, for example, in 58bdf8f9a8.

Thus, the `finalizeSubprogram()` function is called for all definition
subprograms in `DIBuilder::finalize()` because they are stored in the
`AllSubprograms` by the `CreateFunction(isDefinition: true)` call. But
for the declaration subprograms, it should be called manually.

With this commit, `AllSubprograms` is used for holding and finalizing all DISubprograms.