[PCH] Deserializing the DeclContext of a template parameter is not safe
until recursive loading is finished.

Otherwise we may end up with a template trying to deserialize a template
parameter that is in the process of getting loaded.

rdar://13135282

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175329 91177308-0d34-0410-b5e6-96231b3b80d8