Upgrade smallvec to 0.6.7 and use the new may_dangle feature.

[git filter-repo] original commit: rust-lang/rust@ecf6cd4

Author: nnethercote

Cargo.toml

@@ -10,4 +10,4 @@ path = "lib.rs"
 [dependencies]
 bitflags = "1.0"
 rustc_cratesio_shim = { path = "../librustc_cratesio_shim" }
-smallvec = { version = "0.6.5", features = ["union"] }
+smallvec = { version = "0.6.7", features = ["union", "may_dangle"] }