[OpenCL] Override supported OpenCL extensions with -cl-ext option

Summary:
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
extensions:

  // 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
extensions:

  // only fp64 will be supported
  clang -cc1 -triple spir-unknown-unknown -cl-ext=-all,+cl_khr_fp64

Patch by asavonic (Andrew Savonichev).

Reviewers: joey, yaxunl

Subscribers: yaxunl, bader, Anastasia, cfe-commits

Differential Revision: https://reviews.llvm.org/D23712

llvm-svn: 285700