[OpenCL] Add generic type handling for builtin functions

Generic types are an abstraction of type sets.  It mimics the way
functions are defined in the OpenCL specification.  For example,
floatN can abstract all the vector sizes of the float type.

This allows to
 * stick more closely to the specification, which uses generic types;
 * factorize definitions of functions with numerous prototypes in the
   tablegen file; and
 * reduce the memory impact of functions with many overloads.

Patch by Pierre Gondois and Sven van Haastregt.

Differential Revision: https://reviews.llvm.org/D65456

llvm-svn: 369253