[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.

llvm-svn: 145752