Add __CLC_ prefix to all macro definitions in headers

libclc was defining and undefing GENTYPE and several other macros with
common names in its header files.  This was preventing applications from
defining macros with identical names as command line arguments to the
compiler, because the definitions in the header files were masking the
macros defined as compiler arguements.

Reviewed-by: Aaron Watry <awatry@gmail.com>
llvm-svn: 185838