[flang] Changes responding to comments on my previous pull request. Specifically --

parse-tree.cc
 - Cleaned up the use of "const" in several declarations.
check-do-stmt.cc
 - Replaced uses of parser::Messages with SemanticsContext.
 - Removed unused "==" operator from the DoStmtContext class.
 - Reduced the size of the GetBounds function by calling GetLoopControl().
 - Changed the warning message for REAL DO controls to not mention standard
   extensions.
check-do-stmt.h
 - Restored the forward reference to Fortran::parser::DoConstruct and removed
   the include of parse-tree.h
dosemantics*.f90
 - Removed extraneous references to the "RUN" command.
test_errors.sh
 - Simplified and generalized the extraction of the OPTIONS specification.

Original-commit: flang-compiler/f18@04a071276648e505f6f958739239e6f7f5870ff4
Reviewed-on: https://github.com/flang-compiler/f18/pull/478
Tree-same-pre-rewrite: false