[NPM] Resolve llvmGetPassPluginInfo to the plugin being loaded

Dynamically loaded plugins for the new pass manager are initialised by
calling llvmGetPassPluginInfo. This is defined as a weak symbol so that
it is continually redefined by each plugin that is loaded. When loading
a plugin from a shared library, the intention is that
llvmGetPassPluginInfo will be resolved to the definition in the most
recent plugin. However, using a global search for this resolution can
fail in situations where multiple plugins are loaded.

Currently:

* If a plugin does not define llvmGetPassPluginInfo, then it will be
  silently resolved to the previous plugin's definition.

* If loading the same plugin twice with another in between, e.g. plugin
  A/plugin B/plugin A, then the second load of plugin A will resolve to
  llvmGetPassPluginInfo in plugin B.

* The previous case can also occur when a dynamic library defines both
  NPM and legacy plugins; the legacy plugins are loaded first and then
  with `-fplugin=A -fpass-plugi...