[clang] [NFC][clang] Remove superfluous header files after refactor in #132252 (PR #132495)

[Min-Yih Hsu via cfe-commits]

mshockwave wrote:

> If only `CGBuiltin.cpp` and `ARM.cpp` is touched/updated, I see a large difference

This. IMHO I think quality-of-life improvements on incremental build -- especially for LLVM developers -- is already a big thing that can justify this patch by itself.

Regarding the marginal difference on clean builds: my guess is that compilers might spend an even longer time on another source file(s) which dominate the total compilation time. 
To find out if this is true, IIRC clang's `-ftime-trace` can breakdown the time it spends on each input source files. It requires some data collection from the trace file though.

https://github.com/llvm/llvm-project/pull/132495