refactor(@schematics/angular): update vendored TypeScript to 3.9.3

Author: alan-agius4

packages/schematics/angular/third_party/github.com/Microsoft/TypeScript/BUILD.bazel

@@ -1,7 +1,7 @@
 load("//tools:defaults.bzl", "ts_library")
 
-# files fetched on 2020-01-09 from
-# https://github.com/Microsoft/TypeScript/tree/v3.7.4
+# files fetched on 2020-05-20 from
+# https://github.com/Microsoft/TypeScript/tree/v3.9.3
 licenses(["notice"])  # Apache 2.0
 
 ts_library(