Represent debug information compression type fully

Represent debug information compression type fully

This is tied with the LLVM side of the change to expose the debug
information compression types to clang.  We now track the compression
type as an enumeration rather than a boolean.  We still use the same
value (GNU) that we did previously.  This is in preparation to support
passing down the compression type and switch it based on the command
line.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305039 91177308-0d34-0410-b5e6-96231b3b80d8