[OpenCL] Make OpenCL Builtins added according to the right version.

Currently we only have OpenCL 2.0 Builtins i.e. pipes or address space conversions.

They have to be added only in the version 2.0 compilation mode to make the identifiers
available for use in the other versions.

Review: http://reviews.llvm.org/D20249
llvm-svn: 274509