[flang] Tighten rules to resolve procedure as intrinsic procedure

2 Bug fixes:

- Do not resolve procedure as intrinsic if they appeared in an
  EXTERNAL attribute statement (one path was not considering this flag)

- Emit an error if a procedure resolved to be an intrinsic function
  (resp. subroutine) is used as a subroutine (resp. function).
  Lowering was attempted while the evaluate::Expression for the
  call was missing without any errors.

1 behavior change:

- Do not implicitly resolve subroutines (resp. functions) as intrinsics
  because their name is the name of an intrinsic function (resp.
  subroutine). Add justification in documentation.

Reviewed By: klausler, tskeith

Differential Revision: https://reviews.llvm.org/D90049