[AccelTable] Provide abstraction for emitting DWARF5 accelerator tables.

When emitting the DWARF accelerator tables from dsymutil, we don't have
a DwarfDebug instance and we use a custom class to represent Dwarf
compile units. This patch adds an interface AccelTableWriterInfo to
abstract these from the Dwarf5AccelTableWriter, so we can have a custom
implementation for this in dsymutil.

Differential revision: https://reviews.llvm.org/D49031

llvm-svn: 336529