Fix error for indy string concat with a non-String return type

[Maran23]

Summary

For an assignment to a non-String variable, such as Object v = "a"; v += "b";, javac emits an indy string concatenation whose call type returns Object.
The GraalVM OutlineIndyStringConcatPlugin copied that return type.
Then in OutlinedSBMethod.buildGraph, due to Object, the method when into SubstrateSBConcatFactory instead and the image build failed.
(With should not reach here: unexpected input could not be handled: class java.lang.Object).

The return type is now always String, as a string concatenation always produces a String.
Then, the code will instead go to the SubstrateStringConcatFactory, which is correct and will work correctly.

Related Issues

Fixes: #14533

Testing

• Tested with my reproducer in the issue
• Wrote a test which confirms the fix

Documentation

• No documentation update needed. Wrote a javadoc comment in the test.

Contributor Checklist

• I have read the contribution guide.
• I have the right to contribute the submitted material under the project terms.
• I have updated tests and documentation where appropriate.
• If I used a coding assistant, I remain responsible for the entire contribution and have reviewed it accordingly.

[Maran23]

Fixed in #14545 -- as mentioned in #14533, so closing.