Use llvm::is_contained where appropriate (NFC)

Summary:
This patch replaces std::find with llvm::is_contained where
appropriate.

Reviewers: efriedma, nhaehnle

Reviewed By: nhaehnle

Subscribers: arsenm, jvesely, nhaehnle, hiraditya, rogfer01, kerbowa, llvm-commits, vkmr

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84489