[OpenCL] Remove PIPE_RESERVE_ID_VALID_BIT from opencl-c.h

Summary:
PIPE_RESERVE_ID_VALID_BIT is implementation defined, so lets not keep it in the header. 

Previously the topic was discussed here: https://reviews.llvm.org/D32896 

Reviewers: Anastasia, yaxunl

Reviewed By: Anastasia

Subscribers: cfe-commits, asavonic, bader

Differential Revision: https://reviews.llvm.org/D52658

llvm-svn: 345051