Compute 'this' correctly for block in lambda.

Using CurFuncDecl is both correct and simple compared to crawling
the DeclContexts of the block.

Fixes <rdar://problem/14415072>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186210 91177308-0d34-0410-b5e6-96231b3b80d8