This an 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
