Switch CI to use Clang (#28)

Summary:
GCC 7.4 (which is what GitHub Actions supports) is having trouble
compiling the latest trunk, even though we think it is valid C++14.
Pull Request resolved: facebookincubator/fbjni#28

Test Plan: OSS CI passes: https://github.com/facebookincubator/fbjni/commit/cb99a001c4d40e571ae2af5edd1753d9aa2ecba9/checks?check_suite_id=376297298

Reviewed By: swolchok

Differential Revision: D19239659

Pulled By: dreiss

fbshipit-source-id: 6e4724fa10f1367fdacf5f2f9a520dd6d98b30bf

first-party/fbjni/scripts/run-host-tests.sh

@@ -17,6 +17,7 @@ set -exo pipefail
 
 BASE_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )/.."
 CMAKE=$ANDROID_HOME/cmake/3.10.2.4988404/bin/cmake
+export CXX=clang++
 
 mkdir -p "$BASE_DIR/host-build-cmake"
 cd "$BASE_DIR/host-build-cmake"