Kill FunctionDecl's IsCopyAssignment bit; it duplicated what could

Kill FunctionDecl's IsCopyAssignment bit; it duplicated what could
already be determined by isCopyAssignmentOperator(), and was set too
late in the process for all clients to see the appropriate
value. Cleanup only; no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@114916 91177308-0d34-0410-b5e6-96231b3b80d8