[clang][DebugInfo] Emit debuginfo for non-constant case value

Currently, clang does not emit debuginfo for the switch stmt
case value if it is an enum value. For example,
  $ cat test.c
  enum { AA = 1, BB = 2 };
  int func1(int a) {
    switch(a) {
    case AA: return 10;
    case BB: return 11;
    default: break;
    }
    return 0;
  }
  $ llvm-dwarfdump test.o | grep AA
  $
Note that gcc does emit debuginfo for the same test case.

This patch added such a support with similar implementation
to CodeGenFunction::EmitDeclRefExprDbgValue(). With this patch,
  $ clang -g -c test.c
  $ llvm-dwarfdump test.o | grep AA
                  DW_AT_name    ("AA")
  $

Differential Revision: https://reviews.llvm.org/D134705