[Attributor] Add helper class to compose two structured deduction.

Summary: This patch introduces a generic way to compose two structured deductions.  This will be used for composing generic deduction with `MustBeExecutedExplorer` and other existing generic deduction.

Reviewers: jdoerfert, sstefan1

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66645

llvm-svn: 374060