Introduce __builtin_nontemporal_store and __builtin_nontemporal_load.

Summary:
Currently clang provides no general way to generate nontemporal loads/stores.
There are some architecture specific builtins for doing so (e.g. in x86), but
there is no way to generate non-temporal store on, e.g. AArch64. This patch adds
generic builtins which are expanded to a simple store with '!nontemporal'
attribute in IR.

Differential Revision: http://reviews.llvm.org/D12313

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247104 91177308-0d34-0410-b5e6-96231b3b80d8