ASTReader.h: Prune \param ImportLoc according to r202560. [-Wdocumentation]

FIXME: Give appropriate \param to llvm::ArrayRef<SubmoduleID> Overrides.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202589 91177308-0d34-0410-b5e6-96231b3b80d8