PCH: fix a crash caused by a circular deserialization dependency

We started by trying to deserialize decltype(func-param) in a trailing return
type, which causes the function parameter decl to be deserialized, which pulls
in the function decl, which pulls the function type, which pulls the same
decltype() in the return type, and then we crashed.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197644 91177308-0d34-0410-b5e6-96231b3b80d8