When importing a FunctionProtoType::ExtProtoInfo, its ExceptionSpecDecl can point to the

FunctionDecl that we are importing the FunctionProtoType for, in which case we'll have
infinite recursion when importing.

Initially create a FunctionProtoType with null ExceptionSpecDecl/ExceptionSpecTemplate and
update the type in ASTNodeImporter::VisitFunctionDecl after the FunctionDecl has been created.

llvm-svn: 164450