Unify handling of position info in AST, part 1

Unify handling of position info in AST, part 1

* AstNode now has a position info.
* Removed various ad-hoc position infos from subclasses (most of which had it).
* Position is always set with the constructor, instead of later.
* Take care to use kNoPosition in the right spots, to not crash the debugger.

Still to do:

* Eliminate Conditional::then/else_position and WhileStatement::condition_position.
* Make CaseClause a proper AstNode and eliminate its custom position.
* If possible, eliminate all uses of kNoPosition.

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/24076007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17183 ce2b1a6d-e550-0410-aec6-3dcde31c8c00