[OpenCL] Add NULL to standards prior to v2.0.

NULL was undefined in OpenCL prior to version 2.0. However, the
language specification states that "macro names defined by the C99
specification but not currently supported by OpenCL are reserved
for future use". Therefore, application developers cannot redefine
NULL.

The change is supposed to resolve inconsistency between language
versions. Currently there is no apparent reason why NULL should
be kept undefined.

Patch by Topotuna (Justas Janickas)!

Differential Revision: https://reviews.llvm.org/D105988