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_test.go

@@ -6,6 +6,56 @@ import (
 	"testing"
 )
 
+var firstUniqueElementsTestCases = []struct {
+	in  []string
+	out []string
+}{
+	{
+		in:  []string{"a"},
+		out: []string{"a"},
+	},
+	{
+		in:  []string{"a", "b"},
+		out: []string{"a", "b"},
+	},
+	{
+		in:  []string{"a", "a"},
+		out: []string{"a"},
+	},
+	{
+		in:  []string{"a", "b", "a"},
+		out: []string{"a", "b"},
+	},
+	{
+		in:  []string{"b", "a", "a"},
+		out: []string{"b", "a"},
+	},
+	{
+		in:  []string{"a", "a", "b"},
+		out: []string{"a", "b"},
+	},
+	{
+		in:  []string{"a", "b", "a", "b"},
+		out: []string{"a", "b"},
+	},
+	{
+		in:  []string{"liblog", "libdl", "libc++", "libdl", "libc", "libm"},
+		out: []string{"liblog", "libdl", "libc++", "libc", "libm"},
+	},
+}
+
+func TestFirstUniqueElements(t *testing.T) {
+	for _, testCase := range firstUniqueElementsTestCases {
+		out := firstUniqueElements(testCase.in)
+		if !reflect.DeepEqual(out, testCase.out) {
+			t.Errorf("incorrect output:")
+			t.Errorf("     input: %#v", testCase.in)
+			t.Errorf("  expected: %#v", testCase.out)
+			t.Errorf("       got: %#v", out)
+		}
+	}
+}
+
 var lastUniqueElementsTestCases = []struct {
 	in  []string
 	out []string