Fix #21721: make case TypeBlock(_, _) not match non-type Block

[felher]

TypeBlocks are represented as normal Blocks in the Quotes API's implementation. The current TypeTest for TypeBlock is exactly the same as the TypeTest for Block, which means that case TypeBlock(_, _) "matches" every block.

The implementation of unapply on TypeBlockModule, however, gives back (List[TypeDef], TypeTree). It constructs the List[TypeDef] by mapping over every statement of the block, trying to turn it into a TypeDef by using a match with the pattern

  case alias: TypeDef => alias

This seems fine since TypeBlocks are supposed to be just a list of TypeDefs followed by a type as the last expression. Since the TypeTest matches any Block and not only Blocks that are TypeBlocks, the statements can be anything, not just TypeDefs, which lets the whole case TypeBlock(_, _) pattern die with a MatchError.

This commit fixes the problem by making the TypeTest check whether the Block is a type (which in turns checks whether the Blocks expression is a type)

This is my first PR, please let me know if I should change anything or am completely going about fixing this the wrong way, etc.

Also, I could not run the complete test suite on my machine, it died even without making any changes. But I could at least run the positive compilation tests.

Closes #21721

[felher]

I'm a bit unsure whether the failure of the CI has anything to do with the change proposed in this PR.

On my machine, scala3-bootstrapped/testOnly dotty.tools.dotc.CompilationTests runs through fine. No problem like

Error:  Test dotty.tools.dotc.CompilationTests.runAll failed: java.lang.AssertionError: Run test failed, but should not, reasons:
Error:  encountered 1 test failure(s):
Error:    - test 'tests\run\lambda-serialization-gc.scala' timed out, took 179.28 sec
Error:      at dotty.tools.vulpix.ParallelTesting$CompilationTest.checkPass(ParallelTesting.scala:1257)
Error:      at dotty.tools.vulpix.ParallelTesting$CompilationTest.checkRuns(ParallelTesting.scala:1221)
Error:      at dotty.tools.dotc.CompilationTests.runAll(CompilationTests.scala:168)
Error:      ...

I got:

[info] Test dotty.tools.dotc.CompilationTests.runAll started
[=======================================>] completed (1806/1806, 0 failed, 94s)

[hamzaremmal]

Hi @felher, the CI failure has nothing to do with the proposed change. I've restarted the failed job.