[code-reflection] RFR: Erase receiver type before calling Gen::binaryQualifier

[Maurizio Cimadamore]

I found an issue where `Gen::binaryQualifier` was called on a non-denotable reveiver type. This led to a crash, as `binaryQualifier` ended up creating a new symbol whose owner was the non-denotable receiver and, at least for some symbols, we check in the constructor that the owner can't be a type-variable.

The solution is to make sure that the receiver type (`site`) passed to `binaryQualifier` is always erased, which is also the assumption `Gen` makes (obviously).

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/50/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=50&range=00
  Stats: 44 lines in 2 files changed: 42 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/babylon/pull/50.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/50/head:pull/50

PR: https://git.openjdk.org/babylon/pull/50