Dedup exported flags from dependencies

Soong command lines have gotten very long due to hidl modules
reexporting lots of libraries.  Dedup the include dir flags.

Test: m -j checkbuild
Change-Id: I6ada1251012da42344e2c00ae66001a649023d2c

cc/cc.go

@@ -1054,6 +1054,9 @@ func (c *Module) depsToPaths(ctx android.ModuleContext) PathDeps {
 		}
 	})
 
+	// Dedup exported flags from dependencies
+	depPaths.Flags = firstUniqueElements(depPaths.Flags)
+
 	return depPaths
 }
 
@@ -1175,6 +1178,23 @@ func vendorMutator(mctx android.BottomUpMutatorContext) {
 	}
 }
 
+// firstUniqueElements returns all unique elements of a slice, keeping the first copy of each
+// modifies the slice contents in place, and returns a subslice of the original slice
+func firstUniqueElements(list []string) []string {
+	k := 0
+outer:
+	for i := 0; i < len(list); i++ {
+		for j := 0; j < k; j++ {
+			if list[i] == list[j] {
+				continue outer
+			}
+		}
+		list[k] = list[i]
+		k++
+	}
+	return list[:k]
+}
+
 // lastUniqueElements returns all unique elements of a slice, keeping the last copy of each
 // modifies the slice contents in place, and returns a subslice of the original slice
 func lastUniqueElements(list []string) []string {