[flang] Better error recovery for missing THEN in IF construct

[klausler]

Summary

Presented with "IF (...)" with no following tokens in the statement,
diagnose a missing "THEN" instead of complaining about all of the
possible action statement initial tokens that could have been there
for a non-construct IF statement.

Fixes https://github.com/llvm/llvm-project/issues/62299.

[PeteSteinfeld]

All builds and test correctly and looks good.

flang/lib/Parser/executable-parsers.cpp

 // R1137 else-stmt -> ELSE [if-construct-name]
 // R1138 end-if-stmt -> END IF [if-construct-name]
 TYPE_CONTEXT_PARSER("IF construct"_en_US,
     construct<IfConstruct>(
         statement(construct<IfThenStmt>(maybe(name / ":"),
-            "IF" >> parenthesized(scalarLogicalExpr) / "THEN")),
+            "IF" >> parenthesized(scalarLogicalExpr) /
+                    recovery("THEN"_tok, lookAhead(endOfStmt)))),
         block,
         many(construct<IfConstruct::ElseIfBlock>(
             unambiguousStatement(construct<ElseIfStmt>(
                 "ELSE IF" >> parenthesized(scalarLogicalExpr),
                 recovery("THEN"_tok, ok) >> maybe(name))),

flang/test/Parser/missing-then.f90

+! RUN: not %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+!CHECK: expected 'THEN'
+!CHECK-NOT: expected 'PAUSE'
+if (.TRUE.)
+!CHECK: expected 'THEN'
+else if (.FALSE.)
+end if
+end