[clang] [clang][driver] Use TheDriver.ClangExecutable in SetInstallDir to point to the actual install folder (PR #68091)

[Liviu Ionescu via cfe-commits]

ilg-ul wrote:

> it's quite possible that someone has ended up depending on the previous de facto behaviour.

Then we have to be super creative and find a better solution.

How about changing the logic, and where `Driver.InstalledDir` is used, if the desired file/folder is not found (like header or library), to search again in `Driver.ClangExecutable`?

I don't know how complicated the implementation might be, but, at first sight, it should maintain compatibility with curent  use cases that create a full toolchain folder and also prevent silently falling back to system headers/libraries (like in my use case).


https://github.com/llvm/llvm-project/pull/68091