Make `AppendZero` a class member instead of an argument to
`GetOrAddStringOffset` to reflect the intended usage that for a given
`StringToOffsetTable`, all strings must use the same value of
`AppendZero`.
Modify `EmitStringTableDef` to drop the `Indent` argument as its always
set to `""`, and to fail if it's called for a table with
non-null-terminated strings.
This mainly transitions the LLVM intrinsic string table from character
emission to string literal emission, which I confirmed happens for me
locally.
I moved the guts of StringToOffsetTable to a cpp file so I could move
the `EmitLongStrLiterals` cl::opt global to a non-vague linkage home in
the `TableGen` library. I had to add missing FormatVariadic.h includes
to account for moving other includes to a cpp file.
