RFR: 8329951: `var` emits deprecation warnings that do not point to the file or position

[Archie Cobbs]

This PR fixes a bug that causes certain warnings to be emitted without source file location information.

Consider this example:

import java.util.*;
import java.util.function.*;
public class Example {

    @SuppressWarnings("deprecation")
    List<Observable> m1() { return null; }

    void m2() {
        for (var o : m1()) { }
    }
}

the compiler outputs this:

$ javac -Xlint:deprecation Example.java 
warning: [deprecation] Observable in java.util has been deprecated
1 warning

but it should instead output this:

Example.java:9: warning: [deprecation] Observable in java.util has been deprecated
        for (var o : m1()) { }
             ^
1 warning

The bug happens because the "synthetic" type that is installed dynamically for implicitly typed variables is not given a source code position. This makes some sense (?) because that type doesn't literally appear in the variable declaration; however, certain error messages expect to be able to point to the type portion of a variable declaration, so it also causes this bug (and presumably similar variants). To fix this, we just copy the position of the variable declaration itself to the synthetic type.

However, this change necessitated another change: The fix for [JDK-8200199](https://bugs.openjdk.org/browse/JDK-8200199) added back in 2018 relies on the fact that the position of an implicitly typed variable is always null, so this change breaks that fix. But since then, a new method `JCVariableDecl.declaredUsingVar()` was added, so we can just use that test instead. This also makes the code a little clearer.

-------------

Commit messages:
 - Fix bug where some warnings didn't have a source file position.

Changes: https://git.openjdk.org/jdk/pull/23683/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23683&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329951
  Stats: 33 lines in 4 files changed: 30 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23683.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23683/head:pull/23683

PR: https://git.openjdk.org/jdk/pull/23683