Don't call android.PathForModuleSrc(ctx).String() to get ModuleDir

ctx.ModuleDir is much cheaper than android.PathForModuleSrc(ctx).String().

Test: builds
Change-Id: I81819088d9564b06a0336a59a45f2b110b0bd9d6
diff --git a/cc/compiler.go b/cc/compiler.go
index 5bed8a7..490d3cc 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -339,7 +339,7 @@
 // per-target values, module type values, and per-module Blueprints properties
 func (compiler *baseCompiler) compilerFlags(ctx ModuleContext, flags Flags, deps PathDeps) Flags {
 	tc := ctx.toolchain()
-	modulePath := android.PathForModuleSrc(ctx).String()
+	modulePath := ctx.ModuleDir()
 
 	compiler.srcsBeforeGen = android.PathsForModuleSrcExcludes(ctx, compiler.Properties.Srcs, compiler.Properties.Exclude_srcs)
 	compiler.srcsBeforeGen = append(compiler.srcsBeforeGen, deps.GeneratedSources...)