Xcode project cleanups.

- Don't have any header files claim to be part of the lldb-core target. If they are part of the lldb-core target then any file can just #include the header file name without the prefix (#include "Foo.h") when the cmake/make/other builds would require a full path (#include "lldb/Core/Foo.h"). This will help make sure the builds succeed on all platforms when changes are made on MacOSX.
- Add the Hexagon dynamic loader to the DynamicLoader plug-in folder so it gets compiled in MacOSX. There was a recent build bot failure that wasn't caught due to this code not being compile in the MacOSX build

llvm-svn: 240714