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

[klausler]

Summary

The THEN keyword in the "ELSE IF (test) THEN" statement is useless
syntactically, and to omit it is a common error (at least for me!)
that has poor error recovery. This patch changes the parser to
cough up a simple "expected 'THEN'" and still recognize the rest of
the IF construct.

[clementval]

LGTM

flang/lib/Evaluate/check-expression.cpp

 }
 
 bool IsConstantExprHelper::operator()(const ProcedureRef &call) const {
-  // LBOUND, UBOUND, and SIZE with DIM= arguments will have been reritten
+  // LBOUND, UBOUND, and SIZE with DIM= arguments will have been rewritten
   // into DescriptorInquiry operations.
   if (const auto *intrinsic{std::get_if<SpecificIntrinsic>(&call.proc().u)}) {
     if (intrinsic->name == "kind" ||

flang/lib/Parser/executable-parsers.cpp

         many(construct<IfConstruct::ElseIfBlock>(
             unambiguousStatement(construct<ElseIfStmt>(
                 "ELSE IF" >> parenthesized(scalarLogicalExpr),
-                "THEN" >> maybe(name))),
+                recovery("THEN"_tok, ok) >> maybe(name))),
             block)),
         maybe(construct<IfConstruct::ElseBlock>(
             statement(construct<ElseStmt>("ELSE" >> maybe(name))), block)),

flang/test/Parser/elseif-then.f90

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