Fix anonymous union parsing in Natvis for LLDB

[iridinite]

Consider the following example code:

int main(int argc, char *argv[])
{
	struct Foo
	{
		union
		{
			int a;
			float b;
		};
	};

	Foo f;
	f.b = 1.0f;

	__builtin_debugtrap();
}

When breaking at the debug trap, the following MI sequence is exchanged when using LLDB-MI (reformatted for readability):

1022-var-create - - "f" --thread 1 --frame 0
1022^done,name="var2",numchild="1",value="{...}",type="Foo",thread-id="1",has_more="0"

1025-var-list-children --all-values "var2" 0 1000
1025^done,numchild="1",children=[
    child={name="var2.$0",exp="",numchild="2",type="Foo::(anonymous union)",thread-id="1",value="{...}",has_more="0"}
],has_more="0"

1027-var-list-children --all-values "var2.$0" 0 1000
1027^done,numchild="2",children=[
    child={name="var2.$0.a",exp="a",numchild="0",type="int",thread-id="1",value="1065353216",has_more="0"},
    child={name="var2.$0.b",exp="b",numchild="0",type="float",thread-id="1",value="1",has_more="0"}
],has_more="0"

The anonymous union in Foo is listed by LLDB-MI as an unnamed child (no expression) and with type name Foo::(anonymous union). The current check for anonymous unions only accounts for the syntax flavor that GDB uses, and so when using LLDB, anonymous unions incorrectly get flagged as NodeType.Field by MIEngine. This in turn affects FullName() which will return an incorrect expression if the members of the union are also compounds.

This change ensures that the LLDB flavor of anonymous unions get flagged properly.