Track the result of evaluating a computed noexcept specification on the

Track the result of evaluating a computed noexcept specification on the
FunctionProtoType.

We previously re-evaluated the expression each time we wanted to know whether
the type is noexcept or not. We now evaluate the expression exactly once.

This is not quite "no functional change": it fixes a crasher bug during AST
deserialization where we would try to evaluate the noexcept specification in a
situation where we have not deserialized sufficient portions of the AST to
permit such evaluation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331428 91177308-0d34-0410-b5e6-96231b3b80d8