PluginManager: Don't cast from void-pointer to pointer-to-function

GCC warns on reinterpret_cast expressions involving a void-pointer
source and a pointer-to-function destination.  Take a detour through
intptr_t to silence it.

Differential Revision: http://reviews.llvm.org/D4626

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213691 91177308-0d34-0410-b5e6-96231b3b80d8