Add a generic "convert-to-llvm" pass delegating to an interface

The multiple -convert-XXX-to-llvm passes are really nice testing tools for
individual dialects, but the expectation is that a proper conversion should
assemble the conversion patterns using `populateXXXToLLVMConversionPatterns()
APIs. However most customers just chain the conversion passes by convenience.

This pass makes it composable more transparently to assemble the required
patterns for conversion to LLVM dialect by using an interface.
The Pass will scan the input and collect all the dialect present, and for
those who implement the `ConvertToLLVMPatternInterface` it will use it to
populate the conversion pattern, and possible the conversion target.

Since these conversions can involve intermediate dialects, or target other
dialects than LLVM (for example AVX or NVVM), this pass can't statically
declare the required `getDependentDialects()` before the pass pipeline
begins. This is worked around by using an extension in the dialectRegistry
that will be invoked for every new loaded dialects in the context. This
allows to lookup the interface ahead of time and use it to query the
dependent dialects.

Differential Revision: https://reviews.llvm.org/D157183