[docs] Mention that we are in the process of removing the legacy PM for the optimization pipeline

And remove references to flags to turn it off.

Reviewed By: nikic, MaskRay

Differential Revision: https://reviews.llvm.org/D123547

Author: aeubanks

clang/include/clang/Driver/Options.td

@@ -1937,8 +1937,8 @@ def fexperimental_isel : Flag<["-"], "fexperimental-isel">, Group<f_clang_Group>
   Alias<fglobal_isel>;
 defm legacy_pass_manager : BoolOption<"f", "legacy-pass-manager",
   CodeGenOpts<"LegacyPassManager">, DefaultFalse,
-  PosFlag<SetTrue, [], "Use the legacy pass manager in LLVM (deprecated, to be removed in a future release)">,
-  NegFlag<SetFalse, [], "Use the new pass manager in LLVM">,
+  PosFlag<SetTrue>,
+  NegFlag<SetFalse>,
   BothFlags<[CC1Option]>>, Group<f_clang_Group>;
 def fexperimental_new_pass_manager : Flag<["-"], "fexperimental-new-pass-manager">,
   Group<f_clang_Group>, Flags<[CC1Option]>, Alias<fno_legacy_pass_manager>;

llvm/docs/NewPassManager.rst

@@ -480,12 +480,9 @@ optimization pipeline (aka the middle-end) works with both the legacy PM and
 the new PM, whereas the backend target-dependent code generation only works
 with the legacy PM.
 
-For the optimization pipeline, the new PM is the default PM. The legacy PM is
-available for the optimization pipeline by setting various compiler/linker
-flags, e.g. ``-flegacy-pass-manager`` for ``clang``.
-
-There will be efforts to deprecate and remove the legacy PM for the
-optimization pipeline in the future.
+For the optimization pipeline, the new PM is the default PM. Using the legacy PM
+for the optimization pipeline is deprecated and there are ongoing efforts to
+remove its usage.
 
 Some IR passes are considered part of the backend codegen pipeline even if
 they are LLVM IR passes (whereas all MIR passes are codegen passes). This