[1.6>1.7] [MERGE #3849 @MSLaguana] Updating CI to use clang 3.9 after CI image change.

Merge pull request #3849 from MSLaguana:updateCICompiler

netci.groovy

@@ -124,7 +124,7 @@ def CreateXPlatBuildTask = { isPR, buildType, staticBuild, machine, platform, co
     def staticFlag = staticBuild ? "--static" : ""
     def swbCheckFlag = (platform == "linux" && buildType == "debug" && !staticBuild) ? "--wb-check" : "";
     def icuFlag = (platform == "osx" ? "--icu=/usr/local/opt/icu4c/include" : "")
-    def compilerPaths = (platform == "osx") ? "" : "--cxx=/usr/bin/clang++-3.8 --cc=/usr/bin/clang-3.8"
+    def compilerPaths = (platform == "osx") ? "" : "--cxx=/usr/bin/clang++-3.9 --cc=/usr/bin/clang-3.9"
     def buildScript = "bash ./build.sh ${staticFlag} -j=`${numConcurrentCommand}` ${buildFlag} " +
                       "${swbCheckFlag} ${compilerPaths} ${icuFlag} ${customOption} ${extraBuildParams}"
     def testScript = "bash test/runtests.sh \"${testVariant}\""