[PowerPC] Clean up VK_PPC_TOC... names

This is another minor cleanup; to bring enum names in line
with the corresponding @modifier names, this renames:

  VK_PPC_TOC -> VK_PPC_TOCBASE
  VK_PPC_TOC_ENTRY -> VK_PPC_TOC16

No code change intended.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184491 91177308-0d34-0410-b5e6-96231b3b80d8