[PPC] Fix layering issues between MCTargetDesc and CodeGen

See issue #64166 for more information about the layering issue.

The PPCMCTargetDesc library was including CodeGen headers such as
PPCInstrInfo.h and calling inline functions in them. This doesn't work
in the Bazel build, and is error-prone. If the inline function moves to
a cpp file, it will result in linker errors.

To address the issue, I moved several inline functions to
PPCMCTargetDesc.cpp, and declared them in the PPC namespace in
PPCMCTargetDesc.h, which seemed like the most straightforward fix.

Differential Revision: https://reviews.llvm.org/D156488