Remove excessive padding from the struct CallStackFrame

The struct CallStackFrame is in lib/AST/ExprConstant.cpp
inside anonymous namespace.
This diff reorders the fields and removes excessive padding.
Test plan: make -j8 check-clang

Differential revision: https://reviews.llvm.org/D23901


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281907 91177308-0d34-0410-b5e6-96231b3b80d8