[clang] [clang][DependencyScanning] Implementation of `CompilerInstanceWithContext` to Improve By-Name Queries (PR #164345)

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

I discussed this offline with @jansvoboda11 and I will look into two more things:

1. Diagnostics and error handling. Jan caught it that we are not using the fake file buffer. I experimented with a few different buffer sizes and things are working all right when we don't have errors. I will add some tests with diagnostics and errors. The dependency scanning step needs to handle errors better. Maybe we can get rid of the fake buffer if possible. 
2. I will recover the C++ interface of by-name lookup but use the `CompilerInstanceWithContext` to implement it. This way, it is easier to stage with Swift. 

https://github.com/llvm/llvm-project/pull/164345