[PassManagerBuilder] Remove global extension when a plugin is unloaded

This commit fixes PR39321.

GlobalExtensions is not guaranteed to be destroyed when optimizer plugins are unloaded. If it is indeed destroyed after a plugin is dlclose-d, the destructor of the corresponding ExtensionFn is not mapped anymore, causing a call to unmapped memory during destruction.

This commit guarantees that extensions coming from external plugins are removed from GlobalExtensions when the plugin is unloaded if GlobalExtensions has not been destroyed yet.

Differential Revision: https://reviews.llvm.org/D71959