Ensure CLANG_PLUGIN_SUPPORT is compatible with llvm_add_library.
Fixes an issue noted in D111100.
Differential Revision: https://reviews.llvm.org/D119199
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
Jameson Nash