[lldb/Core] Remove more duplicate code in PluginManager (NFCI)

The PluginManager contains a lot of duplicate code. I already removed a
bunch of it by introducing the templated PluginInstance class, and this
is the next step. The PluginInstances class combines the mutex and the
vector and implements the common operations.

To accommodate plugin instances with additional members it is possible
to access the underlying vector and mutex. The methods to query these
fields make use of that.

Differential revision: https://reviews.llvm.org/D74816