swift234

Move 'Playground' into LangOptions and out of FrontendOptions and IRGenOptions.

Also, use 'Playground' to control the behavior of ignored expressions
(which are not an error because they are displayed in the playground log).

This is preparation for LLDB no longer passing 'DebuggerSupport' for a
playground <rdar://problem/18090611>. 'DebuggerSupport' now only applies
to REPL-like contexts and enables identifiers beginning with $, special
rules for parsing top-level code, ignored expressions (like playgrounds),
and the @LLDBDebuggerSupport attribute.

Besides ignored expressions, 'Playground' enables the playground transformation
and provides an entry point for debugger initialization.

Note that this is a bit insincere---many of the options controlled by both
'Playground' and 'DebuggerSupport' really only apply to the main source file
or main module. If/when we add back support for source file imports, we'll
need to revisit all of LangOptions and see which of them should /really/
apply to /everything/ in the ASTContext.

Swift SVN r21384