[flang] Remove the IF specific context classes; in these cases they

added no value. Change several error messages to better say
what is expected.  Adjust tests for new messages.  Reformat
with llvm7 clang-format.

Original-commit: flang-compiler/f18@706b506e26ac675ac443c5b616abda5c4cbd0523
Reviewed-on: https://github.com/flang-compiler/f18/pull/356
Tree-same-pre-rewrite: false