RFR: 8268850: AST model for 'var' variables should more closely adhere to the source code

[Jan Lahoda]

Currently, if a local variable is declared using `var`, or if a lambda parameter has an inferred (missing) type, the AST model is a bit weird. The variable starts with `null` type, and the type is then replaced with a synthetic AST representing the real type. This (draft) PR is attempting to clean this up a little:
- variables declared with `var` have `VarTypeTree` as the type. This is a new AST node that represents `var`. Positions are expected to be exact w.r.t. what is in the source code.
- lambda parameters with inferred type would have `null` as the type. This is consistent with majority of the places in the AST where something is not part of the source code
- for explicit parameters, the type is the real type, as before

This is not a compatible change, but I tried to partially write two clients, and it feels it wasn't too bad, and (hopefully) the new state is cleaner/more consistent.

There are some consistency comments:

- when the inferred types are inaccessible, javac (currently) sometimes produces a compile-time error, and sometimes does not; after reading the JLS, the error is indeed mandated in some cases, and not in others. `test/langtools/tools/javac/lvti/VarAccessibility.java` shows the cases
- when the inferred types have potential for various types of warnings, javac (currently) sometimes produces those warnings, and sometimes not. The decision here is mostly implementation driven. I looked at the possibility to always produce the warnings, but there's https://bugs.openjdk.org/browse/JDK-8013222, from which it seems the original intent was to not produce the warnings. So, this PR currently will not produce warnings on the inferred types.

I was considering adding something similar to get JCVarDecl.DeclKind, but (based on the experience from the clients), it didn't feel strongly needed, as the type is determinable from `getType()`.

Any insights?

CC @cushon

Please also review:
- CSR: https://bugs.openjdk.org/browse/JDK-8376736

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Depends on: https://git.openjdk.org/jdk/pull/29687

Commit messages:
 - Cleanup
 - Adjusting tests.
 - Merge branch 'JDK-8371683' into JDK-8268850-vartype-ast+JDK-8371683
 - Cleanup.
 - Fixing highlighting.
 - Fixing tests.
 - Suppressing warnings.
 - Adjusting accessibility errors and warnings.
 - Adjusting to current mainline changes.
 - Merge remote-tracking branch 'upstream-ssh/master' into JDK-8268850-vartype-ast
 - ... and 12 more: https://git.openjdk.org/jdk/compare/79986247...d3f16c8a

Changes: https://git.openjdk.org/jdk/pull/29369/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8268850
  Stats: 911 lines in 33 files changed: 798 ins; 38 del; 75 mod
  Patch: https://git.openjdk.org/jdk/pull/29369.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29369/head:pull/29369

PR: https://git.openjdk.org/jdk/pull/29369