Fix uninitialized read of CalleeWithThisReturn.

Initialize CalleeWithThisReturn to 0 in the constructor.
Also revert r170815 since checking CalleeWithThisReturn is faster.

PR15598


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178053 91177308-0d34-0410-b5e6-96231b3b80d8