A few more tweaks to the blocks AST representation:

A few more tweaks to the blocks AST representation:
  - BlockDeclRefExprs always store VarDecls
  - BDREs no longer store copy expressions
  - BlockDecls now store a list of captured variables, information about
    how they're captured, and a copy expression if necessary

With that in hand, change IR generation to use the captures data in
blocks instead of walking the block independently.

Additionally, optimize block layout by emitting fields in descending
alignment order, with a heuristic for filling in words when alignment
of the end of the block header is insufficient for the most aligned
field.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125005 91177308-0d34-0410-b5e6-96231b3b80d8