Add a new DynamicLoader plugin that uses SPI that are in development

Add a new DynamicLoader plugin that uses SPI that are in development
for the fall (northern hemisphere) 2016 Darwin platforms to learn
about loaded images, instead of reading dyld internal data structures.
These new SPI don't exist on older releases, and new packets are
needed from debugserver to use them (those changes are already committed).

I had to change the minimum deployment target for debugserver in the xcode
project file to macOS 10.10 so that debugserver will use the
[[NSProcessInfo processInfo] operatingSystemVersion]
call in MachProcess::GetOSVersionNumbers to get the operarting system
version # -- this API is only available in macOS 10.10 and newer
("OS X Yosemite", released Oct 2014).  If we have many people building
llvm.org lldb on older systems still, we can back off on this for the
llvm.org sources.

There should be no change in behavior with this commit, either to
older darwin systems or newer darwin systems.

For now the new DynamicLoader plugin is never activated - I'm forcing
the old plugin to be used in DynamicLoaderDarwin::UseDYLDSPI.
I'll remove that unconditional use of the old plugin soon, so the
newer plugin is used on the newest Darwin platforms.

<rdar://problem/25251243>

llvm-svn: 276254