Add Imagination Technologies to the vendors in llvm::Triple

Summary: This is a pre-requisite for supporting the mips-img-linux-gnu triple in clang.

Differential Revision: http://reviews.llvm.org/D4435


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212626 91177308-0d34-0410-b5e6-96231b3b80d8