[clang][deps] NFC: Rename scanning CompilerInstance

The dependency scanner works with multiple instances of `Compiler{Instance,Invocation}`. From names of the variables/members, their purpose is not obvious.

This patch gives a distinct name to the `CompilerInstance` that's used to run the implicit build during dependency scan.

Depends on D111724.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D111725