RFR: 7903600: Add more tests for unsupported types

[Jorn Vernee]

Add more tests for unsupported types. See the new TestUnsupportedTypes.java

While working on this, I also noticed several cases of dead code: `FunctionalInterfaceScanner` is completely unused, and `InMemoryJavaCompiler::jfoFromByteArray` and `InMemoryJavaCompiler::jfoFromString` (and its caller in `OutputFactory`) are unused as well. I've removed them in this PR.

Changes in this PR:
- I'm passing in the error stream PrintWriter that we create in the test all the way down to UnsupportedFilter, so that it can write to it instead of `Sytstem.err`. Then we can check what was written in the test. 
- I've added warning print outs for the different reasons why we are skipping declarations. Some of these were missing.
- I've removed a check for ValueLayouts that are larger than 8 bytes in `visitVariable`. If the type of a variable is an unsupported primitive, we already filter it out earlier on in the method, so there's no need to check again.
- I've added a missing `Skip.with` in `visitVariable`, so that we will filter out (global) variables with unsupported types.
- I've added skipping of typedefs when they have an unsupported type or a type that does not have a layout. For the latter, I've also had to remove a test case that check if we generated a typedef class for an undefined struct type. Since the struct is undefined, I believe we should not generate a typedef binding class for that case.

The new test covers all the unsupported type cases that are handled by TestUnsupportedTypes.

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Commit messages:
 - polish test
 - skip typedef with unsupported type
 - improve test coverage of UnsupportedFilter
 - remove dead code

Changes: https://git.openjdk.org/jextract/pull/152/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=152&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903600
  Stats: 342 lines in 10 files changed: 170 ins; 151 del; 21 mod
  Patch: https://git.openjdk.org/jextract/pull/152.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/152/head:pull/152

PR: https://git.openjdk.org/jextract/pull/152