[NFC] Rename ThinLTOPhase to ThinOrFullLTOPhase and move it from PassBuilder.h

to Pass.h.

In some compiler passes like SampleProfileLoaderPass, we want to know which
LTO/ThinLTO phase the pass is in. Currently the phase is represented in enum
class PassBuilder::ThinLTOPhase, so it is only available in PassBuilder and
it also cannot represent phase in full LTO. The patch extends it to include
full LTO phases and move it from PassBuilder.h to Pass.h, then it is much
easier for PassBuilder to communiate with each pass about current LTO phase.

Differential Revision: https://reviews.llvm.org/D94613