[OpenCL] opencl-c.h: use uint/ulong consistently

Most places already seem to use the short spelling instead of
'unsigned int/long', so perform the following substitutions:

  s/unsigned int /uint /g
  s/unsigned long /ulong /g

This simplifies completeness comparisons against OpenCLBuiltins.td.

Differential Revision: https://reviews.llvm.org/D120032