[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/main.cpp

@@ -57,7 +57,7 @@ int main(int argc, char const *argv[]) {
   f.i = 1;
   f.j = 0;
   f.k = 1;
-    } 
+    }
   } clang_example;
 
   class B {
@@ -70,6 +70,18 @@ int main(int argc, char const *argv[]) {
     uint32_t d_a : 1;
   } derived;
 
+  union union_with_bitfields {
+      unsigned int a : 8;
+      unsigned int b : 16;
+      unsigned int c : 32;
+      unsigned int x;
+  } uwbf;
+
+  union union_with_unnamed_bitfield {
+   unsigned int : 16, a : 24;
+   unsigned int x;
+  } uwubf;
+
   lba.a = 2;
 
   lbb.a = 1;
@@ -89,5 +101,8 @@ int main(int argc, char const *argv[]) {
   derived.b_a = 2;
   derived.d_a = 1;
 
+  uwbf.x = 0xFFFFFFFF;
+  uwubf.x = 0xFFFFFFFF;
+
   return 0; // Set break point at this line.
 }