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

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 #62299.

Differential Revision: https://reviews.llvm.org/D150783

flang/lib/Parser/executable-parsers.cpp

@@ -305,7 +305,8 @@ TYPE_CONTEXT_PARSER(
 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>(

flang/test/Parser/missing-then.f90

@@ -0,0 +1,8 @@
+! 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