Build binder from llvm source download.
Build binder from llvm source download.

Update build.py to build from llvm source download, rather than git checkout.
This change significantly reduces the required disc space and checkout time for
a binder build by eliminating the full git clone of the llvm and clang.

Updates build invocation to *only* build binder, rather than the full
collection of clang and llvm tools, reducing the total build time.

RosettaCommons/binder,RosettaCommons/binder,RosettaCommons/binder