[flang] Format label scope is independent of block scope

Compilation of the following program currently generates a warning message:

        i = 1
        if (i .eq. 0) then
          write(6, 200) i
200       format (I8)
        end if
        write(6, 200) i
      end

x.f90:6:9: Label '200' is not in scope
          write(6, 200) i
          ^^^^^^^^^^^^^^^

Whereas branch targets must conform to the Clause 11.1.2.1 program
requirement "Transfer of control to the interior of a block from
outside the block is prohibited, ...", this doesn't apply to format
statement references.

Author: klausler

flang/lib/Semantics/resolve-labels.cpp

@@ -935,6 +935,12 @@ void CheckScopeConstraints(const SourceStmtList &stmts,
           parser::MessageFormattedText{
               "Label '%u' was not found"_err_en_US, SayLabel(label)});
     } else if (!InInclusiveScope(scopes, scope, target.proxyForScope)) {
+      // Clause 11.1.2.1 prohibits transfer of control to the interior of a
+      // block from outside the block, but this does not apply to formats.
+      if (target.labeledStmtClassificationSet.test(
+              TargetStatementEnum::Format)) {
+        continue;
+      }
       context.Say(position,
           parser::MessageFormattedText{
               "Label '%u' is not in scope"_en_US, SayLabel(label)});

flang/test/Semantics/io07.f90

@@ -17,6 +17,11 @@
 2011 format(:2L2)
 2012 format(2L2 : 2L2)
 
+     write(*,2013) 'Hello'
+     if (2+2.eq.4) then
+2013   format(A10) ! ok to reference outside the if block
+     endif
+
      ! C1302 warnings; no errors
 2051 format(1X3/)
 2052 format(1X003/)