Add --compilation_mode support in transitions

Add support for converting //command_line_option:compilation_mode from
Java value to Starlark value so that it can be read in a transition.

Fixes --compilation_mode in #10690, but not the general case.

src/main/java/com/google/devtools/build/lib/analysis/config/CompilationMode.java

@@ -13,13 +13,15 @@
 // limitations under the License.
 package com.google.devtools.build.lib.analysis.config;
 
+import com.google.devtools.build.lib.syntax.Printer;
+import com.google.devtools.build.lib.syntax.StarlarkValue;
 import com.google.devtools.common.options.EnumConverter;
 
 /**
  * This class represents the debug/optimization mode the binaries will be
  * built for.
  */
-public enum CompilationMode {
+public enum CompilationMode implements StarlarkValue {
 
   // Fast build mode (-g0).
   FASTBUILD("fastbuild"),
@@ -47,4 +49,9 @@ public Converter() {
       super(CompilationMode.class, "compilation mode");
     }
   }
+
+  @Override
+  public void repr(Printer printer) {
+    printer.append(toString());
+  }
 }