[clang][test] Refine clang machine-function-split tests.

This CL includes two changes:
1. moved clang backend-warnings test cases from Driver/ to CodeGen/.
2. removed multiple `cd "$(dirname "%t")"` and replaced with `-o %t`.

Reviewed By: maskray (Fangrui Song)
Differential Revision: https://reviews.llvm.org/D157565