Replace more parse tree references to "thing" and "value()" with usage
of the parser::Unwrap<> template function.
Add parser::UnwrapRef<> as an alias for DEREF(Unwrap<>()).

Bring the typed expression representation of a coindexed reference up to
F'2023, which removed some restrictions that had allowed the current
representation to suffice for older revisions of the language. This new
representation is somewhat more simple -- it uses a DataRef as its base,
so any subscripts in a part-ref can be represented as an ArrayRef there.

Update the code that creates the CoarrayRef, and add more checking to
it, as well as actually capturing any STAT=, TEAM=, & TEAM_NUMBER=
specifiers that might appear. Enforce the constraint that the part-ref
must have subscripts if it is an array. (And update a pile of
copied-and-pasted test code that lacked such subscripts.)

Add a test for the SYNC IMAGES statement, and add a check for invalid
image numbers.

Add remaining checking for the FORM TEAM statement, complete and enable
a test.

Like DO CONCURRENT and CRITICAL constructs, control flow into and out of
a CHANGE TEAM construct is disallowed.

Add checks for the LOCK statement, and complete and enable their tests.

The event variable in an EVENT POST/WAIT statement can be a coarray
reference, and need not be an entire coarray.

Variables and potential subobject components with EVENT_TYPE/LOCK_TYPE
must be coarrays, unless they are potential subobjects nested within
coarrays or pointers.

Switch to common::visit more call sites.

Test plan: ninja check-all

Add `notify-type` to `iso_fortran_env` module. Add `notify-wait-stmt` to
the parser and add checks for constraints on the statement, `C1177` and
`C1178`, from the Fortran 2023 standard. Add three semantics tests for
`notify-wait-stmt`.

The soon-to-be-published next revision of the ISO Fortran language standard
contains a couple of breaking changes to previous specifications that may cause
existing programs to silently change their behavior.

For the change that introduces automatic reallocation of deferred length
allocatable character scalar variables when they appear as the targets
of internal WRITE statements, as IOMSG=/ERRMSG= variables, as outputs
of INQUIRE specifiers, or as INTENT(OUT) arguments to intrinsic
procedures, this patch adds an optional portability warning.

Differential Revision: https://reviews.llvm.org/D154242