This adds a cc1 flag to enable constructor homing but doesn't turn on debug
info if it wasn't enabled already (which is what using
-debug-info-kind=constructor does). This will be used for testing, and won't
be needed anymore once ctor homing is used as default / merged into =limited.
Bug to enable ctor homing: https://bugs.llvm.org/show_bug.cgi?id=46537
Differential Revision: https://reviews.llvm.org/D85799
Amy Huang