This commit adds support for using new-style pass manager pass plugins from lld.
The commit touches some different parts of the source tree, mainly a configuration option is added to libLTO where an optional list of new-style pass plugins are added. If that list (vector) isn't empty and the user has requested to use a custom pass pipeline, then the pass plugins will be loaded into the passbuilder, using the same code as opt uses when loading custom passes.
Then lld is modified to take a new command line parameter --lto-newpm-load-pass-plugin which can be given multiple times to make lld load a pass plugin.
I also had to modify the CMakeLists.txt of lld to export its symbols for dynamic linking.
To be able to add tests for this functionality I had to add a sample new-style pass to llvm/lib/Transforms/HelloNewPM. The code for that plugin is taken from the article Writing an LLVM Pass in 2018 written by Minh-Yi Hsu,
I also added a test for opts loading of new pass plugins, since it didn't have one.
Of course I'm open to splitting the changes into different ones, but since I didn't want to send a patch without adding tests I made a larger patch.