[Tooling] add a Heuristic field indicating that a CompileCommand was guessed.

Summary:
Use cases:
 - a tool that dumps the heuristic used for each header in a project can
   be used to evaluate changes to the heuristic
 - we want to expose this information to users in clangd as it affects
   accuracy/reliability of editor features
 - express interpolation tests more directly

Reviewers: ilya-biryukov, klimek

Subscribers: ioeric, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60194

llvm-svn: 357770