[flang] Collect names of internal subprograms of a MainProgram

... as is done for other internal subprograms.

Original-commit: flang-compiler/f18@079f45d51b64bcee1197c65201dc66d14ee7c08c
Reviewed-on: https://github.com/flang-compiler/f18/pull/167
Tree-same-pre-rewrite: false