RFR: Fix erasure of null-restricted types

[Maurizio Cimadamore]

There are two issues with erasing null-restricted types:
* not all the code paths in `Types::erasure` drop null restrictions
* for some types like `String`, `Types::erasure` treats erasure as a no-op

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1870/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1870&range=00
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1870.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1870/head:pull/1870

PR: https://git.openjdk.org/valhalla/pull/1870