Rewrite record layout for ms_struct structs.

The old implementation of ms_struct in RecordLayoutBuilder was a
complete mess: it depended on complicated conditionals which didn't
really reflect the underlying logic, and placed a burden on users of
the resulting RecordLayout. This commit rips out almost all of the
old code, and replaces it with simple checks in
RecordLayoutBuilder::LayoutBitField.

This commit also fixes <rdar://problem/14252115>, a bug where class
inheritance would cause us to lay out bitfields incorrectly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185018 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CodeGen/tbaa-struct.cpp b/test/CodeGen/tbaa-struct.cpp
index 6d593a3..9320a83 100644
--- a/test/CodeGen/tbaa-struct.cpp
+++ b/test/CodeGen/tbaa-struct.cpp
@@ -70,5 +70,5 @@
 // CHECK: [[TS2]] = metadata !{i64 0, i64 1, metadata !{{.*}}, i64 4, i64 2, metadata !{{.*}}, i64 8, i64 4, metadata !{{.*}}, i64 12, i64 1, metadata !{{.*}}, i64 16, i64 4, metadata {{.*}}, i64 20, i64 4, metadata {{.*}}}
 // (offset, size) = (0,8) char; (0,2) char; (4,8) char
 // CHECK: [[TS3]] = metadata !{i64 0, i64 8, metadata !{{.*}}, i64 0, i64 2, metadata !{{.*}}, i64 4, i64 8, metadata !{{.*}}}
-// CHECK: [[TS4]] = metadata !{i64 0, i64 1, metadata [[CHAR]], i64 1, i64 1, metadata [[CHAR]], i64 2, i64 1, metadata [[CHAR]]}
+// CHECK: [[TS4]] = metadata !{i64 0, i64 1, metadata [[CHAR]], i64 1, i64 4, metadata [[INT]], i64 1, i64 1, metadata [[CHAR]], i64 2, i64 1, metadata [[CHAR]]}
 // CHECK: [[TS5]] = metadata !{i64 0, i64 1, metadata [[CHAR]], i64 4, i64 4, metadata [[INT]], i64 4, i64 1, metadata [[CHAR]], i64 5, i64 1, metadata [[CHAR]]}