Add -flang flag to the test-release.sh script

The flag is off by default.

llvm/utils/release/test-release.sh

@@ -41,6 +41,7 @@ do_lld="yes"
 do_lldb="no"
 do_polly="yes"
 do_mlir="yes"
+do_flang="no"
 BuildDir="`pwd`"
 ExtraConfigureFlags=""
 ExportBranch=""
@@ -172,6 +173,9 @@ while [ $# -gt 0 ]; do
         -no-mlir )
             do_mlir="no"
             ;;
+        -flang )
+            do_flang="yes"
+            ;;
         -help | --help | -h | --h | -\? )
             usage
             exit 0
@@ -261,6 +265,9 @@ fi
 if [ $do_mlir = "yes" ]; then
   projects="$projects mlir"
 fi
+if [ $do_flang = "yes" ]; then
+  projects="$projects flang"
+fi
 
 # Go to the build directory (may be different from CWD)
 BuildDir=$BuildDir/$RC