RFR: 8283237: CallSite should be a sealed class [v4]

[liach]

> Change `CallSite` to a sealed class, as `CallSite` is an abstract class which does not allow direct subclassing by users per its documentation. Since I don't have a JBS account, I posted the content for the CSR in a GitHub Gist at https://gist.github.com/150d5aa7f8b13a4deddf95969ad39d73 and wish someone can submit a CSR for me.

liach has updated the pull request incrementally with one additional commit since the last revision:

  Revert "Remove homemade callsite from test"
  
  This reverts commit 1bcd779f677420326bf365c3580ceab5a6b5e3fa.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7840/files
  - new: https://git.openjdk.java.net/jdk/pull/7840/files/1bcd779f..c0a10f6e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7840&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7840&range=02-03

  Stats: 119 lines in 3 files changed: 113 ins; 2 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7840.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7840/head:pull/7840

PR: https://git.openjdk.java.net/jdk/pull/7840