[clang] [lldb] Use CLANG_RESOURCE_DIR more consistently (PR #103388)

[Kim Gräsman via cfe-commits]

kimgr wrote:

> > Is there an out-of-tree scenario where CLANG_RESOURCE_DIR needs to be replaced with something else at runtime, i.e. a real use-case for the optional CustomResourceDir optional argument I removed?
> 
> @kimgr I have also looked for this and I haven't found an use-case where this would be appropriate. In fact, I think it's wrong to not take `CLANG_RESOURCE_DIR` as part of the output from `driver::Driver::GetResourcesPath()` because that would generate wrong results for builds that do set `CLANG_RESOURCE_DIR`, e.g. Fedora's builds.

@tuliom I think it might be possible that a standalone tool would want to use its own executable path and a hardcoded relative `CustomResourceDir`, but then maybe it would be better to just build the relevant path without calling `GetResourcesPath`.

https://github.com/llvm/llvm-project/pull/103388