gn build: set HAVE_VCS_VERSION_INC while building Version.cpp

And belatedly merge r353268 / r353269.

test/Driver/wasm-toolchain-lto.c currently requires getLLVMRevision()
to return something non-empty to pass. That's hopefully temporary,
but making Version.cpp work is probalby a good idea regardless.
(Note its contents will by default usually be out-of-date, because
llvm_allow_tardy_revision defaults to false.)