Set dwarf version to 4 in test/producer.f90

Signed-off-by: Paul Osmialowski <pawel.osmialowski@arm.com>

Author: pawosm-arm

test/debug_info/producer.f90

@@ -1,7 +1,7 @@
 !RUN: %flang -gdwarf-4 -S -emit-llvm %s -o - | FileCheck %s
 
 !CHECK: !DICompileUnit(language: DW_LANG_Fortran90
-!CHECK-SAME: flags: "'+flang -gdwarf-4 -S -emit-llvm 
+!CHECK-SAME: flang -gdwarf-4 -S -emit-llvm
 
 program main
 end program main