[Sema] Correct typos in LHS, RHS before building a binop expression.

Specifically, typo correction should be done before dispatching between
different kinds of binary operations like pseudo-object assignment,
overloaded binary operation, etc.

Without this change we hit an assertion

    Assertion failed: (!LHSExpr->hasPlaceholderType(BuiltinType::PseudoObject)), function CheckAssignmentOperands

when in Objective-C we reference a property without `self` and there are
2 equally good typo correction candidates: ivar and a class name. In
this case LHS expression in `BuildBinOp` is

    CXXDependentScopeMemberExpr
    `-TypoExpr

and instead of handling Obj-C property assignment as pseudo-object
assignment, we call `CreateBuiltinBinOp` which corrects typo to

    ObjCPropertyRefExpr '<pseudo-object type>'

but cannot handle pseudo-objects and asserts about it (indirectly,
through `CheckAssignmentOperands`).

rdar://problem/33102722

Reviewers: rsmith, ahatanak, majnemer

Reviewed By: ahatanak

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D37322

llvm-svn: 313323