Add a clang-7 build target with --strict-warnings

[extended tests]

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9450)

.travis.yml

@@ -43,6 +43,10 @@ matrix:
           dist: trusty
           compiler: clang
           env: CONFIG_OPTS="--strict-warnings -D__NO_STRING_INLINES no-deprecated" BUILDONLY="yes"
+        - os: linux
+          dist: bionic
+          compiler: clang
+          env: CONFIG_OPTS="--strict-warnings -D__NO_STRING_INLINES no-deprecated" BUILDONLY="yes"
         - os: linux
           addons:
               apt: