Add kotlinc 1.4.20 to third_party

Contents of README.google:

Name: Kotlin Compiler
URL: https://github.com/JetBrains/kotlin/
Version: 1.4.20
Revision: NA
License: Apache License Version 2.0

Bug: 174292935
Change-Id: I0cec1aab43e6860a7387525db363e8f7aeb0747b
diff --git a/.gitignore b/.gitignore
index bee945e..7d1f237 100644
--- a/.gitignore
+++ b/.gitignore
@@ -99,6 +99,8 @@
 third_party/kotlin/kotlin-compiler-1.3.41
 third_party/kotlin/kotlin-compiler-1.3.72.tar.gz
 third_party/kotlin/kotlin-compiler-1.3.72
+third_party/kotlin/kotlin-compiler-1.4.20.tar.gz
+third_party/kotlin/kotlin-compiler-1.4.20
 third_party/kotlinx-coroutines-1.3.6.tar.gz
 third_party/kotlinx-coroutines-1.3.6
 third_party/nest/*