[OpenCL] Include opencl-c.h by default as a clang module

Include opencl-c.h by default as a module to utilize the automatic AST caching mechanism of clang modules.

Add an option -finclude-default-header to enable default header for OpenCL, which is off by default.

Differential Revision: http://reviews.llvm.org/D20444

llvm-svn: 273191