Debug Info: Store the SDK in the DICompileUnit.

This is another intermediate step for PR44213
(https://bugs.llvm.org/show_bug.cgi?id=44213).

This stores the SDK *name* in the debug info, to make it possible to
`-fdebug-prefix-map`-replace the sysroot with a recognizable string
and allowing the debugger to find a fitting SDK relative to itself,
not the machine the executable was compiled on.

rdar://problem/51645582