LLVM IR Dialect: separate the conversion tool from the conversion pass

Originally, the conversion to the LLVM IR dialect had been implemented as pass.
The common conversion infrastructure was factored into DialectConversion from
which the conversion pass inherited.  The conversion being a pass is
undesirable for callers that only need the conversion done, for example as a
part of sequence of conversions or outside the pass manager infrastructure.
Split the LLVM IR Dialect conversion into the conversion proper and the
conversion pass, where the latter contains the former instead of inheriting.
NFC.

PiperOrigin-RevId: 240874740