[OpenCL] Make -cl-ext a driver option.

For generic targets such as SPIR-V clang sets all OpenCL
extensions/features as supported by default. However
concrete targets are unlikely to support all extensions
features, which creates a problem when such generic SPIR-V
binary is compiled for a specific target later on.

To allow compile time diagnostics for unsupported features
this flag is now being exposed in the clang driver.

Differential Revision: https://reviews.llvm.org/D125243