[triton-ext] Pass a single PluginInfo * pointer

[abrown]

This changes how we pass data between Triton plugins and Triton. Instead of implementing multiple functions with various signatures, we collect all the pointers into a single struct PluginInfo and pass this as a pointer across the host-plugin boundary when tritonGetPluginInfo is called. This simplifies things on both the host side (Triton has less ABI surface to worry about) and on the plugin side (less intermediate registration into maps). It also ensures that what we pass is extern "C": we now fully rely on the C ABI, which should avoid a whole category of potential problems down the line.

This approach--passing a single struct across the API--is taken from how LLVM implements plugins. E.g., MLIR's DialectPlugin and PassPlugin do exactly this. Triton's case is a bit more tricky since we can export multiple passes and dialects from a single plugin. But since libraries are loaded via LLVM's
DynamicLibrary::getPermanentLibrary, we expect these plugins to have a lifetime spanning the entire program. In other words, the PluginInfo* pointer would be leaked if we attempted to teardown Triton mid-process (?) but we simply don't do this.

This change also allows us to pass multiple plugins to Triton: we replace TRITON_PASS_PLUGIN_PATH with a colon-separated TRITON_PLUGIN_PATHS. I tested this over in triton-ext and it works as expected.

For the rationale behind these changes, see the Triton Extensions RFCv2 document.

[ThomasRaoux]

Looks like a good clean up to me

[abrown]

Looks like an unrelated build failure... can someone re-run?