[docs] Add note about interaction between clang plugins and -clear-ast-before-backend

[aeubanks]

add header

[thakis]

clang/docs/ClangPlugins.rst

209

this sounds like plugins turning off that flag saves memory

[aeubanks]

reword

[rnk]

Thanks for writing docs, apologies for bikeshedding.

clang/docs/ClangPlugins.rst

207

I'd try to word this as a general recommendation that plugins should run before the main action, rather than documentation that running after the main action has undesired side effects (memory usage, settings).

A first sentence suggestion:

To reduce peak memory usage of the compiler, plugins are recommended to run *before* the main action, which is usually code generation. This is because ... something `-clear-ast-before-backend`

[aeubanks]

reword

[rnk]

lgtm, thanks!

clang/docs/ClangPlugins.rst

 action (i.e. the same as using `-add-plugin`):
 
 .. code-block:: c++
 
   // Automatically run the plugin after the main AST action
   PluginASTAction::ActionType getActionType() override {
     return AddAfterMainAction;
   }
+
+Interaction with ``-clear-ast-before-backend``
+----------------------------------------------
+
+To reduce peak memory usage of the compiler, plugins are recommended to run
+*before* the main action, which is usually code generation. This is because
+having any plugins that run after the codegen action automatically turns off
+``-clear-ast-before-backend``.  ``-clear-ast-before-backend`` reduces peak
+memory by clearing the Clang AST after generating IR and before running IR
+optimizations. Use ``CmdlineBeforeMainAction`` or ``AddBeforeMainAction`` as
+``getActionType`` to run plugins while still benefitting from
+``-clear-ast-before-backend``. Plugins must make sure not to modify the AST,
+otherwise they should run after the main action.
+