DebugInfo: Support for compressed debug info sections

1. When creating a .debug_* section and instead create a .zdebug_ section.
2. When creating a fragment in a .zdebug_* section, make it a compressed fragment.
3. When computing the size of a compressed section, compress the data and use the size of the compressed data.
4. Emit the compressed bytes.

Also, check that only if a section has a compressed fragment, then that
is the only fragment in the section.

Assert-fail if the fragment's data is modified after it is compressed.

Initial review on llvm-commits by Eric Christopher and Rafael Espindola.