[flang] Preserve source location when rewriting parse tree

When a Designator is created from a FunctionReference or
StmtFunctionStmt, copy the source data member too.

Original-commit: flang-compiler/f18@bf91c0630f80a413a374367f0189b7fd21fbab15
Reviewed-on: https://github.com/flang-compiler/f18/pull/422
Tree-same-pre-rewrite: false