[find-all-symbols] Slim SymbolInfo.

Summary:
SymbolInfo has some optional fields, which is a bad-smell
implementation. For now, we

* remove the optional field since we don't need them (we can probably
  add them back if we actually need them in the future)
* make SymbolInfo to be a class.

By this change, the code is more simplified.

Reviewers: klimek

Subscribers: cfe-commits, ioeric, bkramer

Differential Revision: http://reviews.llvm.org/D20095

llvm-svn: 269162