Add the LABEL_KEYED_STRING_DICT type for attributes.

This enables both native and Skylark rules to declare attributes which
have labels/Targets as keys, and have string values.

--
PiperOrigin-RevId: 148365033
MOS_MIGRATED_REVID=148365033
diff --git a/src/main/java/com/google/devtools/build/lib/packages/ProtoUtils.java b/src/main/java/com/google/devtools/build/lib/packages/ProtoUtils.java
index c34ad2e..7c0335d 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/ProtoUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/ProtoUtils.java
@@ -18,6 +18,7 @@
 import static com.google.devtools.build.lib.packages.BuildType.FILESET_ENTRY_LIST;
 import static com.google.devtools.build.lib.packages.BuildType.LABEL;
 import static com.google.devtools.build.lib.packages.BuildType.LABEL_DICT_UNARY;
+import static com.google.devtools.build.lib.packages.BuildType.LABEL_KEYED_STRING_DICT;
 import static com.google.devtools.build.lib.packages.BuildType.LABEL_LIST;
 import static com.google.devtools.build.lib.packages.BuildType.LICENSE;
 import static com.google.devtools.build.lib.packages.BuildType.NODEP_LABEL;
@@ -68,6 +69,7 @@
           .put(TRISTATE, Discriminator.TRISTATE)
           .put(INTEGER_LIST, Discriminator.INTEGER_LIST)
           .put(STRING_DICT_UNARY, Discriminator.STRING_DICT_UNARY)
+          .put(LABEL_KEYED_STRING_DICT, Discriminator.LABEL_KEYED_STRING_DICT)
           .build();
 
   /** Returns the {@link Discriminator} value corresponding to the provided {@link Type}. */