New flag `--incompatible_depset_union` to disable old-style depset merging.

RELNOTES:
  Using `+`, `|` or `.union` on depsets is now deprecated. Please use the new
  constructor instead (see https://docs.bazel.build/versions/master/skylark/depsets.html).
PiperOrigin-RevId: 178262338
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
index 709449a..7f6f6c6 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
@@ -43,6 +43,9 @@
   public abstract boolean incompatibleCheckedArithmetic();
   public abstract boolean incompatibleComprehensionVariablesDoNotLeak();
   public abstract boolean incompatibleDepsetIsNotIterable();
+
+  public abstract boolean incompatibleDepsetUnion();
+
   public abstract boolean incompatibleDictLiteralHasNoDuplicates();
   public abstract boolean incompatibleDisallowDictPlus();
   public abstract boolean incompatibleDisallowKeywordOnlyArgs();
@@ -75,6 +78,7 @@
           .incompatibleCheckedArithmetic(true)
           .incompatibleComprehensionVariablesDoNotLeak(true)
           .incompatibleDepsetIsNotIterable(false)
+          .incompatibleDepsetUnion(false)
           .incompatibleDictLiteralHasNoDuplicates(true)
           .incompatibleDisallowDictPlus(false)
           .incompatibleDisallowKeywordOnlyArgs(true)
@@ -98,6 +102,9 @@
     public abstract Builder incompatibleCheckedArithmetic(boolean value);
     public abstract Builder incompatibleComprehensionVariablesDoNotLeak(boolean value);
     public abstract Builder incompatibleDepsetIsNotIterable(boolean value);
+
+    public abstract Builder incompatibleDepsetUnion(boolean value);
+
     public abstract Builder incompatibleDictLiteralHasNoDuplicates(boolean value);
     public abstract Builder incompatibleDisallowDictPlus(boolean value);
     public abstract Builder incompatibleDisallowKeywordOnlyArgs(boolean value);