RFR: 8264766: ClassCastException during template compilation (Variable can not be cast to Param)

[Joe Wang]

Fixes the addVariable/addParam methods in the SymbolTable to check types before casting.

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Commit messages:
 - 8264766: ClassCastException during template compilation (Variable cannot be cast to Param)

Changes: https://git.openjdk.java.net/jdk/pull/4398/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4398&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264766
  Stats: 70 lines in 2 files changed: 64 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4398.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4398/head:pull/4398

PR: https://git.openjdk.java.net/jdk/pull/4398