[OpenCL] Apply missing restrictions for Blocks in OpenCL v2.0

Applying the following restrictions for block types in OpenCL (v2.0 s6.12.5):
 - __block storage class is disallowed
 - every block declaration must be const qualified and initialized
 - a block can't be used as a return type of a function
 - a blocks can't be used to declare a structure or union field
 - extern speficier is disallowed

Corrected image and sampler types diagnostics with struct and unions.

Review: http://reviews.llvm.org/D16928
llvm-svn: 262616