The new PM is considered stable and many downstream groups have adopted it (some
have adopted it for more than two years). Add -f[no-]legacy-pass-manager to reflect the
fact that it is no longer experimental and the legacy pass manager is something we strive to retire.
In the future, when the legacy PM eventually goes away,
-fno-experimental-new-pass-manager and -flegacy-pass-manager will be removed.
This patch also changes -f[no-]legacy-pass-manager to pass `-plugin-opt={new,legacy}-pass-manager` to the linker (supported by both ld.lld and LLVMgold.so) when -flto/-flto=thin is specified
Reviewed By: aeubanks, rsmith
Differential Revision: https://reviews.llvm.org/D92915
Fangrui Song