Based on usage, the NamedDecl is always set when constructing a CallingContext, but none of the other optional parameters are. Removing the optional parameters, and making the NamedDecl required.

No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203149 91177308-0d34-0410-b5e6-96231b3b80d8