[PCH] Fix crash with valid code, related to anonymous field initializers.

In a certain code-path we were not deserializing an anonymous field initializer correctly,
leading to a crash when trying to IRGen it.

This is a simpler version of a patch by Yunzhong Gao!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182974 91177308-0d34-0410-b5e6-96231b3b80d8