[mlir][Conversion] Store const type converter in ConversionPattern

ConversionPatterns do not (and should not) modify the type converter that they are using.

* Make `ConversionPattern::typeConverter` const.
* Make member functions of the `LLVMTypeConverter` const.
* Conversion patterns take a const type converter.
* Various helper functions (that are called from patterns) now also take a const type converter.

Differential Revision: https://reviews.llvm.org/D157601