Deprecating clang Property

Clang has defaulted to true since 2018. Clang property is deprecated.
Set up a BUILD_BROKEN_CLANG_PROPERTY flag so partners can bypass \
errors from using clang.

Bug: 208980553
Test: m nothing & treehugger
Change-Id: I8fd7d3e6bdbb963279248d3482e8a590238727e3
diff --git a/android/variable.go b/android/variable.go
index 2d7b0bf..7a080fe 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -430,6 +430,7 @@
 
 	ShippingApiLevel *string `json:",omitempty"`
 
+	BuildBrokenClangProperty           bool     `json:",omitempty"`
 	BuildBrokenDepfile                 *bool    `json:",omitempty"`
 	BuildBrokenEnforceSyspropOwner     bool     `json:",omitempty"`
 	BuildBrokenTrebleSyspropNeverallow bool     `json:",omitempty"`