[flang] Add semantics test for ERROR STOP statement

[ngeorge1098]

Add standard conforming statements that use parentheses around the stop-code.
Add standard non-conforming statements that use parentheses around all arguments.

[ngeorge1098]

Add standards-conforming statements for integer expressions
and character phrases. Add standards non-conforming statement
for using quiet= without stop-code. Change language of
comments to properly reflect the language of the standards.

[ktras]

Test looks great, but I would suggest adding non-conforming statements where the stop-code is not of the default integer kind, thus testing C1171 from the Fortran 2018 Standard, and adding a stop-code where it is not of the default char kind, thus testing the definition of one of the options for a stop code, which is scalar-default-char-expr.

[ngeorge1098]

Add standard-non-conforming statements for stop-code integer not being
of default kind and stop-code character not being of default kind.

[ktras]

LGTM

flang/test/Semantics/error_stop.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! This test checks for semantic errors in error stop statements based on the
+! statement specification in section 11.6.7 of the Fortran 2018 standard.
+
+program test_error_stop
+  implicit none
+
+  integer int_code, int_array(1), int_coarray[*], array_coarray(1)[*]
+  character(len=128) char_code, char_array(1), char_coarray[*], non_logical
+  logical bool, logical_array(1), logical_coarray[*], non_integer, non_character
+
+  !___ standard-conforming statement with no optional arguments___
+  error stop
+
+  !___ standard-conforming statement with stop-code argument _____
+  error stop int_code
+  error stop 5
+  error stop char_code
+  error stop 'c'
+  error stop int_array(1)
+  error stop char_array(1)
+  error stop int_coarray
+  error stop int_coarray[1]
+  error stop char_coarray
+  error stop char_coarray[1]
+  error stop array_coarray(1)
+  error stop array_coarray(1)[1]
+
+  !___ standard-conforming statement with all arguments _________
+  error stop int_code, quiet=bool
+  error stop int_code, quiet=logical_array(1)
+  error stop int_code, quiet=logical_coarray
+  error stop int_code, quiet=logical_coarray[1]
+  error stop int_code, quiet=.true.
+
+  !___ non-standard-conforming statement ___
+
+  ! unknown stop-code keyword
+  !ERROR: expected execution part construct
+  error stop code=int_code
+
+  ! invalid stop-code argument typing
+  error stop non_integer
+
+  ! invalid stop-code argument typing
+  error stop non_character
+
+  ! stop-code must be scalar
+  error stop char_array
+
+  ! stop-code must be scalar
+  error stop array_coarray[1]
+
+  ! invalid quiet= argument typing
+  error stop int_code, quiet=non_logical
+
+  ! quiet= variable must be scalar
+  error stop int_code, quiet=logical_array
+
+  ! missing keyword argument for quiet= argument
+  !ERROR: expected execution part construct
+  error stop int_code, bool
+
+  ! unknown keyword argument for quiet= argument
+  !ERROR: expected execution part construct
+  error stop int_code, quiets=bool
+
+  ! missing quiet= variable
+  !ERROR: expected execution part construct
+  error stop int_code, quiet
+
+  ! Too many arguments
+  !ERROR: expected execution part construct
+  error stop int_code, char_code
+
+  ! Repeated quiet= keyword
+  !ERROR: expected execution part construct
+  error stop int_code, quiet=bool, quiet=.true.
+
+  ! Too many arguments
+  !ERROR: expected execution part construct
+  error stop int_code, char_code, quiet=bool
+
+  ! Too many arguments
+  !ERROR: expected execution part construct
+  error stop int_code, quiet=bool, 5
+
+end program test_error_stop