Add DeviceUsesClang to change clang default

This is equivalent to USE_CLANG_PLATFORM_BUILD in the current build
system.

Change-Id: Ifaca0f2639871dac834ef603cfade695191cff11
diff --git a/common/config.go b/common/config.go
index 1340db8..fbd2be9 100644
--- a/common/config.go
+++ b/common/config.go
@@ -214,6 +214,13 @@
 	return *c.ProductVariables.DeviceName
 }
 
+func (c *config) DeviceUsesClang() bool {
+	if c.ProductVariables.DeviceUsesClang != nil {
+		return *c.ProductVariables.DeviceUsesClang
+	}
+	return false
+}
+
 // DeviceOut returns the path to out directory for device targets
 func (c *config) DeviceOut() string {
 	return filepath.Join(c.BuildDir(), "target/product", c.DeviceName())