[IR]PATCH 1/2: Add AsmWriterContext into AsmWriter

AsmWriterContext is a simple compound that stores TypePrinting,
SlotTracker (i.e. "Machine" in AsmWriter), and Module instances -- three
of the most commonly used objects in the AsmWriter infrastructure.
Previously these three objects are passed as separate function arguments
to most of the printer functions in this file. Tidying them up can bring
easier code refactoring on printer functions in the future (e.g. when we
want to pass additional objects to all printer functions).

NOTE: Theoritically, this patch should be NFC.

Differential Revision: https://reviews.llvm.org/D110112