Complete Rewrite of CGRecordLayoutBuilder

CGRecordLayoutBuilder was aging, complex, multi-pass, and shows signs of 
existing before ASTRecordLayoutBuilder.  It redundantly performed many 
layout operations that are now performed by ASTRecordLayoutBuilder and 
asserted that the results were the same.  With the addition of support 
for the MS-ABI, such as placement of vbptrs, vtordisps, different 
bitfield layout and a variety of other features, CGRecordLayoutBuilder 
was growing unwieldy in its redundancy.

This patch re-architects CGRecordLayoutBuilder to not perform any 
redundant layout but rather, as directly as possible, lower an 
ASTRecordLayout to an llvm::type.  The new architecture is significantly 
smaller and simpler than the CGRecordLayoutBuilder and contains fewer 
ABI-specific code paths.  It's also one pass.

The architecture of the new system is described in the comments. For the 
most part, the new system simply takes all of the fields and bases from 
an ASTRecordLayout, sorts them, inserts padding and dumps a record. 
Bitfields, unions and primary virtual bases make this process a bit more 
complicated.  See the inline comments.

In addition, this patch updates a few lit tests due to the fact that the 
new system computes more accurate llvm types than CGRecordLayoutBuilder. 
Each change is commented individually in the review.

Differential Revision: http://llvm-reviews.chandlerc.com/D2795

llvm-svn: 201907