[LLDB] Fix handling of bit-fields in a union

When parsing DWARF and laying out bit-fields we don't properly take into account when they are in a union, they will all have a zero offset.

Differential Revision: https://reviews.llvm.org/D91118

lldb/test/API/lang/cpp/bitfields/TestCppBitfields.py

@@ -46,6 +46,16 @@ class CppBitfieldsTestCase(TestBase):
         self.expect("expr (clang_example.f.a)", VARIABLES_DISPLAYED_CORRECTLY,
                     substrs=['uint64_t', '1'])
 
+        self.expect("expr uwbf",
+            substrs=['a = 255',
+                    'b = 65535',
+                    'c = 4294967295',
+                    'x = 4294967295'] )
+
+        self.expect("expr uwubf",
+            substrs=['a = 16777215',
+                    'x = 4294967295'] )
+
         self.expect(
             "frame variable --show-types lba",
             VARIABLES_DISPLAYED_CORRECTLY,