Pass --crate-type to rustdoc

This supports the corresponding rustc PR. To enable rustdoc to properly
document macros, we mirror the '--crate-type' flag used by rustc.
Currently, all crate types other than 'proc-macro' are ignored by
rustdoc.

Author: Aaron1011

src/cargo/core/compiler/compilation.rs

@@ -140,6 +140,10 @@ impl<'cfg> Compilation<'cfg> {
         if target.edition() != Edition::Edition2015 {
             p.arg(format!("--edition={}", target.edition()));
         }
+        for crate_type in target.rustc_crate_types() {
+            p.arg("--crate-type").arg(crate_type);
+        }
+
         Ok(p)
     }