[PCH] Make sure that the SourceExpr of a OpaqueValueExpr is always initialized
when deserialized, fixing random crashes in libclang.

Also simplifies how OpaqueValueExprs are [de]serialized.
The reader/writer automatically retains pointer equality of sub-statements (when a
statement node is referenced in multiple nodes), so no need to manually handle it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145752 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed