[CodeGen] Refactor AppleAccelTable

Summary:
This commit separates the abstract accelerator table data structure
from the code for writing out an on-disk representation of a specific
accelerator table format. The idea is that former (now called
AccelTable<T>) can be reused for the DWARF v5 accelerator tables
as-is, without any further customizations.

Some bits of the emission code (now living in the EmissionContext class)
can be reused for DWARF v5 as well, but the subtle differences in the
layout of various subtables mean the sharing is not always possible.
(Also, the individual emit*** functions are fairly simple so there's a
tradeoff between making a bigger general-purpose function, and two
smaller targeted functions.)

Another advantage of this setup is that more of the serialization logic
can be hidden in the .cpp file -- I have moved declarations of the
header and all the emission functions there.

Reviewers: JDevlieghere, aprantl, probinson, dblaikie

Subscribers: echristo, clayborg, vleschuk, llvm-commits

Differential Revision: https://reviews.llvm.org/D43285

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325516 91177308-0d34-0410-b5e6-96231b3b80d8

Author: labath