Keep only captured symbols in CodeBlock symbol tables.

Keep only captured symbols in CodeBlock symbol tables.
<https://webkit.org/b/128050>

Discard all uncaptured symbols at the end of codegen since only
the captured ones will be used after that point.

~2MB progression on Membuster OSUS.

Reviewed by Geoffrey Garen.

* bytecode/UnlinkedCodeBlock.h:
(JSC::UnlinkedCodeBlock::setSymbolTable):
* bytecompiler/BytecodeGenerator.cpp:
(JSC::BytecodeGenerator::generate):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@163337 268f45cc-cd09-0410-ab3c-d52691b4dbfc