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

[Kim Gräsman via cfe-commits]

kimgr wrote:

Do we need anything more to make progress with this PR?

My own primary concerns are basically things I can't check myself:

* 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?
* The only call in-tree whose behavior changed is https://github.com/llvm/llvm-project/blob/99b85cae628c1cc5641944290712cd84ccf1f6c8/clang/tools/libclang/CIndexer.cpp#L156. Is there any reason to think libclang should behave differently?

Not sure who can shed light on these questions.

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