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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@374060 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm