Add an SDK attribute to DICompileUnit

This is part of PR44213 https://bugs.llvm.org/show_bug.cgi?id=44213

When importing (system) Clang modules, LLDB needs to know which SDK
(e.g., MacOSX, iPhoneSimulator, ...) they came from. While the sysroot
attribute contains the absolute path to the SDK, this doesn't work
well when the debugger is run on a different machine than the
compiler, and the SDKs are installed in different directories. It thus
makes sense to just store the name of the SDK instead of the absolute
path, so it can be found relative to LLDB.

rdar://problem/51645582

Differential Revision: https://reviews.llvm.org/D75646