TypeSynthetic: Fix a pointer-to-function to void-pointer cast

reinterpret_cast may not convert a pointer-to-function to a
void-pointer.  Take a detour through intptr_t and *then* convert to a
pointer-to-function.

Differential Revision: http://reviews.llvm.org/D4627

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213682 91177308-0d34-0410-b5e6-96231b3b80d8