Auto merge of rust-lang#703 - KyleMayes:clang-sys, r=emilio

Bump clang-sys version to 0.18.0

This update improves detection of versioned instances of `libclang.so` (e.g., `libclang.so.3.9`).

See [here](https://github.com/KyleMayes/clang-sys/tree/db88aea22b3ad67020fab331b42ddc071e598c8c#dependencies) for information on which instance is selected when there are more than one to choose from.

Thanks to @semarie for a pull request implementing this.

Author: bors-servo

Cargo.lock

@@ -44,7 +44,7 @@ quasi_codegen = "0.32"
 [dependencies]
 cexpr = "0.2"
 cfg-if = "0.1.0"
-clang-sys = { version = "0.17.0", features = ["runtime", "clang_3_9"] }
+clang-sys = { version = "0.18.0", features = ["runtime", "clang_3_9"] }
 lazy_static = "0.2.1"
 syntex_syntax = "0.58"
 regex = "0.2"