Expose Clang wrapper's dump usage stats option

This allows us to track how much time is spent in Clang.

Test: TOOLCHAIN_RUSAGE_OUTPUT=/tmp/rusage.txt m
Change-Id: Ib2961904f363bc59bd9d928bb055a96740cb9f17

ui/build/dumpvars.go

@@ -205,6 +205,9 @@ func runMakeProductConfig(ctx Context, config Config) {
 		"CCACHE_SLOPPINESS",
 		"CCACHE_BASEDIR",
 		"CCACHE_CPP2",
+
+		// LLVM compiler wrapper options
+		"TOOLCHAIN_RUSAGE_OUTPUT",
 	}
 
 	allVars := append(append([]string{

ui/build/ninja.go

@@ -169,6 +169,9 @@ func runNinjaForBuild(ctx Context, config Config) {
 			"CCACHE_BASEDIR",
 			"CCACHE_CPP2",
 			"CCACHE_DIR",
+
+			// LLVM compiler wrapper options
+			"TOOLCHAIN_RUSAGE_OUTPUT",
 		}, config.BuildBrokenNinjaUsesEnvVars()...)...)
 	}