Rustdoc support.

Adds `m rustdoc` which generates documentation for all Rust libraries
to $OUT_DIR/soong/rustdoc.

Follow up work:

* Generate an index page that lists all modules.
* Preserve the artifacts so we can have an always-up-to-date go link.

Test: m rustdoc
Bug: None
Change-Id: Id2d6b9cbab5b02e36b575567563d7cc7606b9401
diff --git a/rust/prebuilt.go b/rust/prebuilt.go
index 94fe1e5..49f3c0f 100644
--- a/rust/prebuilt.go
+++ b/rust/prebuilt.go
@@ -103,6 +103,12 @@
 	return srcPath
 }
 
+func (prebuilt *prebuiltLibraryDecorator) rustdoc(ctx ModuleContext, flags Flags,
+	deps PathDeps) android.OptionalPath {
+
+	return android.OptionalPath{}
+}
+
 func (prebuilt *prebuiltLibraryDecorator) compilerDeps(ctx DepsContext, deps Deps) Deps {
 	deps = prebuilt.baseCompiler.compilerDeps(ctx, deps)
 	return deps