[modules] When parsing the base specifiers of a parse-merged class, the current

[modules] When parsing the base specifiers of a parse-merged class, the current
context is the class itself but lookups should be performed starting with the
lookup parent of the class (class and base members don't shadow types from the
surrounding context because they have not been declared yet).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245236 91177308-0d34-0410-b5e6-96231b3b80d8