[OpenCL] Actually activate Frontend/opencl.cl test and fix test bugs

rL275318 added the test Frontend/opencl.cl test, but that test was never actually run because Frontend/lit.local.cfg doesn't contain the '.cl' file suffix.

Once the test is activated, it fails with (unintended) compile errors in the newly added CHECK_INVALID_OPENCL_VERSION checks.

This patch adds the '.cl' file suffix to Frontend/lit.local.cfg to activate the test and fixes the test bug by adding '-fblocks' to the relevant command lines.

Patch by Martin Böhme!

Differential Revision: http://reviews.llvm.org/D22349

llvm-svn: 275405