Run correct clang --version

For the test that uses `clang-9` do the sanity call using `clang-9`
instead of `clang`.

For the test that uses `clang` add a sanity call to `clang --version`.

Backport of 97dc0ea9acdc308cc2cadc11e4787d07ec9b2c79:
  rust-bitcoin/rust-secp256k1#284 (9/9)

Author: tcharding

contrib/test.sh

@@ -59,6 +59,7 @@ fi
 
 # Address Sanitizer
 if [ "$DO_ASAN" = true ]; then
+    clang --version
     cargo clean
     CC='clang -fsanitize=address -fno-omit-frame-pointer'                                        \
     RUSTFLAGS='-Zsanitizer=address -Clinker=clang -Cforce-frame-pointers=yes'                    \