[flang] Supply missing source location for a message

[klausler]

Summary

An error message in semantic analysis of SELECT TYPE type guards
doesn't allow for the possibility of failure from FindSourceLocation(),
and that can happen when the type guard is just an intrinsic type
name like "type is (character)". Deal with it.

[clementval]

LGTM

flang/lib/Semantics/check-select-type.cpp

return common::visit(
                   context_.Say(stmt.source,
                       "If selector is not unlimited polymorphic, "
                       "an intrinsic type specification must not be specified "
                       "in the type guard statement"_err_en_US);
                   typeSpecRetVal = false;
                 }
                 if (spec->category() == DeclTypeSpec::Character &&
                     !guardDynamicType.IsAssumedLengthCharacter()) { // C1160
-                  context_.Say(parser::FindSourceLocation(typeSpec),
+                  auto location{parser::FindSourceLocation(typeSpec)};
+                  context_.Say(location.empty() ? stmt.source : location,
                       "The type specification statement must have "
                       "LEN type parameter as assumed"_err_en_US);
                   typeSpecRetVal = false;
                 }
               } else {
                 const DerivedTypeSpec *derived{spec->AsDerived()};
                 typeSpecRetVal = PassesDerivedTypeChecks(
                     *derived, parser::FindSourceLocation(typeSpec));

flang/test/Semantics/selecttype01.f90

type :: t(n)
     integer, len :: n
   end type
 contains
   subroutine CheckC1160( a )
     class(*), intent(in) :: a
     select type ( a )
       !ERROR: The type specification statement must have LEN type parameter as assumed
       type is ( character(len=10) ) !<-- assumed length-type
+      !ERROR: The type specification statement must have LEN type parameter as assumed
+      type is ( character )
       ! OK
       type is ( character(len=*) )
       !ERROR: The type specification statement must have LEN type parameter as assumed
       type is ( t(n=10) )
       ! OK
       type is ( t(n=*) )   !<-- assumed length-type
       !ERROR: Derived type 'character' not found
       class is ( character(len=10) ) !<-- assumed length-type