CodeGen: handle blocks correctly when inalloca'ed

When using blocks with C++ on Windows x86, it is possible to have the
block literal be pushed into the inalloca'ed parameters.  Teach IRGen to
handle the case properly by extracting the block literal from the
inalloca parameter.  This fixes the use of blocks with C++ on Windows
x86.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@325724 91177308-0d34-0410-b5e6-96231b3b80d8