[COFF] Move section name encoding into BinaryFormat

Large COFF section names are moved into the string table and the
section header field is the offset into the string table encoded in
ASCII for offset smaller than 7 digits and in base64 for larger
offsets.

The operation of taking the string table offsets is done in a few
places in the codebase, so it is helpful to move this operation into
`BinaryFormat` so that it can be shared everywhere it's done.

So this patch takes the implementation of this operation from
`llvm/lib/MC/WinCOFFObjectWriter.cpp` and moves it into `BinaryFormat`.

Reviewed By: jhenderson, rnk

Differential Revision: https://reviews.llvm.org/D118793