TableGen/SearchableTables: Support more generic enums and tables

Summary:
This is essentially a rewrite of the backend which introduces TableGen
base classes GenericEnum, GenericTable, and SearchIndex. They allow
generating custom enums and tables with lookup functions using
separately defined records as the underlying database.

Also added as part of this change:

- Lookup functions may use indices composed of multiple fields.

- Instruction fields are supported similar to Intrinsic fields.

- When the lookup key has contiguous numeric values, the lookup
  function will directly index into the table instead of using a binary
  search.

The existing SearchableTable functionality is internally mapped to the
new primitives.

Change-Id: I444f3490fa1dbfb262d7286a1660a2c4308e9932

Reviewers: arsenm, tra, t.p.northover

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D48013

llvm-svn: 335225