Check if "instrumentation" and "sampling" are set together

Setting these simultaneously is an error.

Test: Check error generation in a test module

Change-Id: I2ee127fa1b7157fa5f4489d2541ff023c64f9ba5
diff --git a/cc/pgo.go b/cc/pgo.go
index 5d1c58a..a99cbad 100644
--- a/cc/pgo.go
+++ b/cc/pgo.go
@@ -124,13 +124,14 @@
 	}
 
 	// Sampling not supported yet
-	//
-	// TODO When sampling support is turned on, check that instrumentation and
-	// sampling are not simultaneously specified
 	if isSampling {
 		ctx.PropertyErrorf("pgo.sampling", "\"sampling\" is not supported yet)")
 	}
 
+	if isSampling && isInstrumentation {
+		ctx.PropertyErrorf("pgo", "Exactly one of \"instrumentation\" and \"sampling\" properties must be set")
+	}
+
 	return true
 }