[clang] Fix initializer_list matching failures with modules

Previously, if a class with a defined public virtual destructor is
declared before including <initializer_list> and initializer_list is
provided via a Clang module, then overload resolution would fail for
std::initializer_list. This is because when Clang sees the virtual
destructor, Clang creates an implicit NamespaceDecl for std to
implicitly declare a std::bad_alloc. That NamespaceDecl is not added to
the translation unit's lookup table, so when the module containing
std::initializer_list is imported later, that module's std NamespaceDecl
can't find the previous std NamespaceDecl during redeclaration lookup,
causing overload resolution to fail.

To fix this, implicitly created std NamespaceDecls are now added to the
lookup map. At the same time, their IdentifierNamespace members are
cleared to prevent regular name lookups from finding it.

Fixes 60929

Reviewed By: ChuanqiXu, #clang-language-wg, inclyc

Differential Revision: https://reviews.llvm.org/D150001