Use LLVM version string

Summary:
macOS builds of LLDB use the bundle version from
`tools/driver/lldb-Info.plist`. That file hasn't been updated since the 4.0
release so the version we print provides no value. I also think that even if it
were up to date, that number has no meaning and displaying the version from the
LLVM tree is more valuable.

I know that Apple folks have some form of override for the clang version to
match the Xcode version, so it'd make sense for them to do the same for LLDB.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D39429

llvm-svn: 317218