Fix safe-init error in TreePickler

When bootstrapping the compiler with the `-Ysafe-init` flag, we would get the following error:

```
[error] -- Error: /*************/dotty/compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala:24:20
[error] 24 |  pickler.newSection(ASTsSection, buf)
[error]    |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |Call method TreePickler.this.pickler.newSection("ASTs",
[error]    |  dotty.tools.dotc.core.tasty.TreePickler.this.buf
[error]    |) on a value with an unknown initialization. Calling trace:
[error]    | -> val treePkl: TreePickler = new TreePickler(this)	[ TastyPickler.scala:85 ]
[error]    |   -> class TreePickler(pickler: TastyPickler) {	[ TreePickler.scala:22 ]
```

This supresses the warning for this case.

Review by @liufengyun

Author: Xavientois

compiler/src/dotty/tools/dotc/core/tasty/TastyPickler.scala

@@ -82,5 +82,5 @@ class TastyPickler(val rootCls: ClassSymbol) {
     all.bytes
   }
 
-  val treePkl: TreePickler = new TreePickler(this)
+  val treePkl: TreePickler = new TreePickler(this: @unchecked)
 }