[ClangIR][Lowering][NFC] Split LowerToLLVM.h from LowerToLLVM.cpp (#1102)

This is a NFC patch that moves declaration from  LowerToLLVM.cpp.

The motivation of the patch is, we hope we can use the abilities from
MLIR's standard dialects without lowering **ALL** clangir operation to
MLIR's standard dialects. For example, currently we have 86 operations
in LowerToLLVM.cpp but only 45 operations under though MLIR. It won't be
easy to add proper lowering for all operation to **different** dialects.

I think the solution may be to allow **mixed** IR. So that we can
lowering CIR to MLIR's standard dialects partially and we can use some
existing analysis and optimizations in MLIR and then we can lower all of
them (the MLIR dialects and unlowered clangir) to LLVM IR. The hybrid IR
is one of the goals of MLIR as far as I know.

NOTE: I completely understand that the DirectlyLLVM pipeline is the
tier-1 pipeline that we want to support. The idea above won't change
this. I just want to offer some oppotunities for the downstream projects
and finally some chances to improve the overall ecosystem.

Author: ChuanqiXu9