[clang] Add plugin ActionType to run command line plugin before main action

Currently we have a way to run a plugin if specified on the command line
after the main action, and ways to unconditionally run the plugin before
or after the main action, but no way to run a plugin if specified on the
command line before the main action.

This introduces the missing option.

This is helpful because -clear-ast-before-backend clears the AST before
codegen, while some plugins may want access to the AST.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D112096