[flang] Fix parsing of WRITE(I+J) with more accurate look-ahead

[klausler]

Summary

The parsing of I/O units uses look-ahead to discriminate between
keywords, variables and expressions as part of distinguishing internal
from external I/O. The look-ahead was inaccurate for variables that
appear as the initial parts of expressions.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/lib/Parser/Fortran-parsers.cpp

TYPE_PARSER(construct<SubscriptTriplet>(
     maybe(subscript), ":" >> maybe(subscript), maybe(":" >> subscript)))
 
 // R925 cosubscript -> scalar-int-expr
 constexpr auto cosubscript{scalarIntExpr};
 
 // R924 image-selector ->
 //        lbracket cosubscript-list [, image-selector-spec-list] rbracket
 TYPE_CONTEXT_PARSER("image selector"_en_US,
-    construct<ImageSelector>("[" >> nonemptyList(cosubscript / !"="_tok),
+    construct<ImageSelector>(
+        "[" >> nonemptyList(cosubscript / lookAhead(space / ",]"_ch)),
         defaulted("," >> nonemptyList(Parser<ImageSelectorSpec>{})) / "]"))
 
 // R926 image-selector-spec ->
 //        STAT = stat-variable | TEAM = team-value |
 //        TEAM_NUMBER = scalar-int-expr
 TYPE_PARSER(construct<ImageSelectorSpec>(construct<ImageSelectorSpec::Stat>(
                 "STAT =" >> scalar(integer(indirect(variable))))) ||
     construct<ImageSelectorSpec>(construct<TeamValue>("TEAM =" >> teamValue)) ||

flang/lib/Parser/io-parsers.cpp

 #include "flang/Parser/parse-tree.h"
 
 namespace Fortran::parser {
 // R1201 io-unit -> file-unit-number | * | internal-file-variable
 // R1203 internal-file-variable -> char-variable
 // R905 char-variable -> variable
 // "char-variable" is attempted first since it's not type constrained but
 // syntactically ambiguous with "file-unit-number", which is constrained.
-TYPE_PARSER(construct<IoUnit>(variable / !"="_tok) ||
+TYPE_PARSER(construct<IoUnit>(variable / lookAhead(space / ",);\n"_ch)) ||
     construct<IoUnit>(fileUnitNumber) || construct<IoUnit>(star))
 
 // R1202 file-unit-number -> scalar-int-expr
 TYPE_PARSER(construct<FileUnitNumber>(scalarIntExpr / !"="_tok))
 
 // R1204 open-stmt -> OPEN ( connect-spec-list )
 TYPE_CONTEXT_PARSER("OPEN statement"_en_US,
     construct<OpenStmt>(