Debug Info: Represent local anonymous unions as anonymous unions

in the debug info. This patch deletes a hack that emits the members
of local anonymous unions as local variables.

Besides being morally wrong, the existing representation using local
variables breaks internal assumptions about the local variables' storage
size.

Compiling

```
   void fn1() {
     union {
       int i;
       char c;
     };
     i = c;
   }

```

with -g -O3 -verify will cause the verifier to fail after SROA splits
the 32-bit storage for the "local variable" c into two pieces because the
second piece is clearly outside the 8-bit range that is expected for a
variable of type char. Given the choice I'd rather fix the debug
representation than weaken the verifier.

Debuggers generally already know how to deal with anonymous unions when
they are members of C++ record types, but they may have problems finding
the local anonymous struct members in the expression evaluator.

rdar://problem/20730771

llvm-svn: 236059