Introduce llvm.noalias.decl intrinsic

The ``llvm.experimental.noalias.scope.decl`` intrinsic identifies where a noalias
scope is declared. When the intrinsic is duplicated, a decision must
also be made about the scope: depending on the reason of the duplication,
the scope might need to be duplicated as well.

Reviewed By: nikic, jdoerfert

Differential Revision: https://reviews.llvm.org/D93039