-Wuninitialized: Split the classification of DeclRefExprs as initialization or

use out of TransferFunctions, and compute it in advance rather than on-the-fly.

This allows us to handle compound assignments with DeclRefExprs on the RHS
correctly, and also makes it trivial to treat const& function parameters as not
initializing the argument. The patch also makes both of those changes.

llvm-svn: 160330