Disable gnu-folding-constant warning.

New clang compiler issue too many such warnings.

Bug: 179065588
Bug: 179309338
Bug: 179278370
Test: make, presubmit
Change-Id: I9948a4b34ec75a01aa24c2fd753722b072ae5202

cc/config/clang.go

@@ -142,6 +142,9 @@ func init() {
 		// Nested and array designated initialization is nice to have.
 		"-Wno-c99-designator",
 
+		// Warnings from clang-12
+		"-Wno-gnu-folding-constant",
+
 		// Calls to the APIs that are newer than the min sdk version of the caller should be
 		// guarded with __builtin_available.
 		"-Wunguarded-availability",