[flang] Folding improvments:

  - Add location to non-fatal error message (context was not given
    a location before folding in semantics).
  - Avoid refolding expression from scratch several times (update GenericExprWrapper
    after folding). So far, warning messages were generated twice per parametere init
    expressions.

Original-commit: flang-compiler/f18@bf7f6cf9cde1c6822b4daeaa44d4f67ffbaef197
Reviewed-on: https://github.com/flang-compiler/f18/pull/468
Tree-same-pre-rewrite: false