[ASTImporter] Fix 'isVirtual()' assert failure while import overridden methods

CXXMethodDecl::isVirtual() count the number of overridden methods.
This assertion is not true before overridden methods are fully loaded.
The body of this CXXMethodDecl can introduce deps on a derived class
which contains a method overriding this method, causing the assertion failure.

ImportOverriddenMethods() is moved before body loading to fix this issue.

Testcase is contributed by Balázs Kéri (balazske)

Differential Revision: https://reviews.llvm.org/D154701