[flang][NFC] Expose FIR to LLVM patterns (#83492)

The FIR dialect has been initiated before many interfaces have been
introduced to MLIR. This patch expose the FIR to LLVM patterns in a
`populateFIRToLLVMConversionPatterns` function. The idea is to be able
to add the `ConvertToLLVMPatternInterface`. This is not directly
possible since the FIR dialect does not currently use the table
infrastructure for its definition.

Follow up patches will move the FIR dialect definition to table gen and
then implement the interface.