Circle CI's clang build to really use clang (#6965)

Summary:
The CircleCI's Clang flavor has a bug that doesn't really use CLANG. Fix it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6965

Test Plan: See CI results.

Reviewed By: pdillinger

Differential Revision: D22025355

fbshipit-source-id: e86922b9152e9f5732e5099d0ce41da9226ff806

.circleci/config.yml

@@ -53,7 +53,9 @@ jobs:
     resource_class: 2xlarge
     steps:
       - checkout # check out the code in the project directory
-      - run: USE_CLANG=1 make all -j32
+      - run: sudo apt-get update -y
+      - run: sudo apt-get install -y clang
+      - run: CC=clang CXX=clang++ V=1 USE_CLANG=1 PORTABLE=1 make all -j32
 
   build-linux-cmake:
     machine: