Add -gdb-set/-gdb-show aggregate-field-names option (MI)

Use this option to print/skip field names (default is on):
```
-var-create var1 * complx
^done,name="var1",numchild="3",value="{i = 3, inner = {l = 3}, complex_ptr = 0x[0-9a-f]+}",type="complex_type",thread-id="1",has_more="0"
-var-create var2 * complx_array
^done,name="var2",numchild="2",value="{[0] = {i = 4, inner = {l = 4}, complex_ptr = 0x[0-9a-f]+}, [1] = {i = 5, inner = {l = 5}, complex_ptr = 0x[0-9a-f]+}}",type="complex_type [2]",thread-id="1",has_more="0"
-gdb-set print aggregate-field-names off
^done
-var-create var3 * complx
^done,name="var3",numchild="3",value="{3,{3},0x[0-9a-f]+}",type="complex_type",thread-id="1",has_more="0"
-var-create var4 * complx_array
^done,name="var4",numchild="2",value="{{4,{4},0x[0-9a-f]+},{5,{5},0x[0-9a-f]+}}",type="complex_type [2]",thread-id="1",has_more="0"
```

llvm-svn: 235807