The clang-analyzer plugins are not linked to a particular tool, so they
can only be compiled if plugins are broadly supported. We could opt
instead to decide whether to link them to specifically against clang or
with undefined symbols, depending on the value of LLVM_ENABLE_PLUGINS,
but we do not currently expect there to be a use case for that rather
niche configuration.
Differential Revision: https://reviews.llvm.org/D119591
Ensure CLANG_PLUGIN_SUPPORT is compatible with llvm_add_library.
Fixes an issue noted in D111100.
Differential Revision: https://reviews.llvm.org/D119199