[clang][deps] NFC: Rename building CompilerInvocation

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 descriptive name to the generated `CompilerInvocation` that can be used to derive the command-line to build a modular dependency.

Depends on D111725.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D111728