Introduce __builtin_nontemporal_store and __builtin_nontemporal_load.


Introduce builtin_nontemporal_store and builtin_nontemporal_load.

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: