[OpenCL] Change type of block pointer for OpenCL

Summary:

For some reason OpenCL blocks in LLVM IR are represented as function pointers.
These pointers do not point to any real function and never get called. Actually
they point to some structure, which in turn contains pointer to the real block
invoke function.
This patch changes represntation of OpenCL blocks in LLVM IR from function
pointers to pointers to `%struct.__block_literal_generic`.
Such representation allows to avoid unnecessary bitcasts and simplifies
further processing (e.g. translation to SPIR-V ) of the module for targets
which do not support function pointers.

Patch by: Alexey Sotkin.

Reviewers: Anastasia, yaxunl, svenvh

Reviewed By: Anastasia

Subscribers: alexbatashev, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58277

llvm-svn: 354337