[OpenCL] Add block argument CodeGen test

r326937 ("[OpenCL] Remove block invoke function from emitted block
literal struct", 2018-03-07) broke block argument handling.  In
particular the commit was causing a crash during code generation, see
the discussion in https://reviews.llvm.org/D43783 .

The offending commit has just been reverted; add a test to avoid
breaking this again in the future.

llvm-svn: 343583