Add lldb to the build

This optionally adds lldb (and clang, which it needs) to the build.

Because rust uses LLVM 7, and because clang 7 is not yet released, a
recent git master version of clang is used.

The lldb that is used includes the Rust plugin.

lldb is only built when asked for, or when doing a nightly build on
macOS.  Only macOS is done for now due to difficulties with the Python
dependency.

src/bootstrap/builder.rs

@@ -461,6 +461,7 @@ impl<'a> Builder<'a> {
                 dist::Rustfmt,
                 dist::Clippy,
                 dist::LlvmTools,
+                dist::Lldb,
                 dist::Extended,
                 dist::HashSign
             ),