This patch adds a command line option '-cl-ext' to control a set of
supported OpenCL extensions. Option accepts a comma-separated list
of extensions prefixed with '+' or '-'.
It can be used together with a target triple to override support for some
// spir target supports all extensions, but we want to disable fp64 clang -cc1 -triple spir-unknown-unknown -cl-ext=-cl_khr_fp64
Special 'all' extension allows to enable or disable all possible
// only fp64 will be supported clang -cc1 -triple spir-unknown-unknown -cl-ext=-all,+cl_khr_fp64