### What changes were proposed in this pull request?
Check more strictly that a field name can be used as a valid Java identifier in `ScalaReflection.serializerFor`
To check that, `SourceVersion` is used so that we need not add reserved keywords to be checked manually for the future Java versions (e.g, underscore, var, yield), .
### Why are the changes needed?
In the current implementation, `enum` is not checked even though it's a reserved keyword.
Also, there are lots of characters and sequences of character including numeric literals but they are not checked.
So we can't get better error message with following code.
```
case class Data(`0`: Int)
Seq(Data(1)).toDF.show
20/04/11 03:24:24 ERROR CodeGenerator: failed to compile: org.codehaus.commons.compiler.CompileException: File 'generated.java', Line 43, Column 1: Expression "value_0 = value_3" is not a type
org.codehaus.commons.compiler.CompileException: File 'generated.java', Line 43, Column 1: Expression "value_0 = value_3" is not a type
...
```
### Does this PR introduce any user-facing change?
Yes. With this change and the code example above, we can get following error message.
```
java.lang.UnsupportedOperationException: `0` is not a valid identifier of Java and cannot be used as field name
- root class: "Data"
...
```
### How was this patch tested?
Add another assertion to existing test case.
Closes#28184 from sarutak/improve-identifier-check.
Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
6cd0bef7fe