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_.

compiler/testData/foreignAnnotations/tests/checkerFramework.kt

@@ -0,0 +1,33 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
+// FILE: A.java
+
+import org.checkerframework.checker.nullness.compatqual.*;
+
+public class A {
+    @NullableDecl public String field = null;
+
+    @NullableDecl
+    public String foo(@NonNullDecl String x, @NullableDecl CharSequence y) {
+        return "";
+    }
+
+    @NonNullDecl
+    public String bar() {
+        return "";
+    }
+
+}
+
+// FILE: main.kt
+
+fun main(a: A) {
+    a.foo("", null)?.length
+    a.foo("", null)<!UNSAFE_CALL!>.<!>length
+    a.foo(<!NULL_FOR_NONNULL_TYPE!>null<!>, "")<!UNSAFE_CALL!>.<!>length
+
+    a.bar().length
+    a.bar()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.length
+
+    a.field?.length
+    a.field<!UNSAFE_CALL!>.<!>length
+}

compiler/testData/foreignAnnotations/tests/checkerFramework.txt

@@ -0,0 +1,13 @@
+package
+
+public fun main(/*0*/ a: A): kotlin.Unit
+
+public open class A {
+    public constructor A()
+    @org.checkerframework.checker.nullness.compatqual.NullableDecl public final var field: kotlin.String?
+    @org.checkerframework.checker.nullness.compatqual.NonNullDecl public open fun bar(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    @org.checkerframework.checker.nullness.compatqual.NullableDecl public open fun foo(/*0*/ @org.checkerframework.checker.nullness.compatqual.NonNullDecl x: kotlin.String, /*1*/ @org.checkerframework.checker.nullness.compatqual.NullableDecl y: kotlin.CharSequence?): kotlin.String?
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}