Recognize Checker Framework declaration annotations.

We are migrating Guava to use these annotations rather than jsr305's
@Nullable. We can't use the Checker Framework's _@Nullable_ yet because
we promise compatibility with Java 7, which doesn't support type
annotations. This is related to but distinct from
https://youtrack.jetbrains.com/issue/KT-21408, which is about a
different jsr305 annotation we use, @ParametersAreNonnullByDefault.

I've also updated some docs to mention Kotlin's existing support for the
Checker Framework _@NonNull_.

third-party/annotations/org/checkerframework/checker/nullness/compatqual/NonNullDecl.java

@@ -0,0 +1,15 @@
+package org.checkerframework.checker.nullness.compatqual;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Java 7 compatibility annotation without dependency on Java 8 classes.
+ *
+ * @see org.checkerframework.checker.nullness.qual.NonNull
+ * @checker_framework.manual #nullness-checker Nullness Checker
+ */
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+public @interface NonNullDecl {}

third-party/annotations/org/checkerframework/checker/nullness/compatqual/NullableDecl.java

@@ -0,0 +1,15 @@
+package org.checkerframework.checker.nullness.compatqual;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Java 7 compatibility annotation without dependency on Java 8 classes.
+ *
+ * @see org.checkerframework.checker.nullness.qual.Nullable
+ * @checker_framework.manual #nullness-checker Nullness Checker
+ */
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+public @interface NullableDecl {}