clover: Re-order includes in invocation.cpp to fix build

clover: Re-order includes in invocation.cpp to fix build

The build was failing because the official CL headers have a few defines,
like: # define cl_khr_gl_sharing 1

Which have the same name as some class members of clang's OpenCLOptions class.
If we include the cl headers first, this breaks the build because the member
names of this class are replaced by the literal 1.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Vedran Miletić <vedran@miletic.net>