Remove incorrect move operations for indexed items

Declare copy operations of the affected classes noexcept to improve
performance of classes that contain them.

The implementations of the removed move constructors were wrong in case
either the source or the destination (but not both) were
disk-reference-counted. The move assignment operators were apparently
correct but, as stated in added @note comments and detailed in this
commit message below, they weren't useful in practice.

Note that the removed IndexedInstantiationInformation's move operations
contained another mistake: they used
standardInstantiationInformationIndex() in place of 0 (see the
implementation of IndexedInstantiationInformation(uint index)).

I have analyzed the calls of copy and move constructors, assignment
operators and swap() specializations for the three classes
(IndexedIdentifier, IndexedQualifiedIdentifier,
IndexedInstantiationInformation) by running statistics-printing-patched
KDevelop several times with kdevelop, kate, ktexteditor and
syntax-highlighting projects loaded. The results:
1. In the move assignment operators almost always either the indices are
   equal or neither operand is DUChain-reference-counted. Only in less
   than 0.5% of calls to IndexedQualifiedIdentifier's move assignment
   operator the indices differ and the left operand is
   reference-counted. The right operand is never reference-counted when
   the indices differ.
2. In the move constructors neither operand is ever reference-counted.
3. No swap() specialization is ever called.
4. Even in the copy special operations, never once both operands were
   reference-counted at the same time when the indices differed (the
   same is true for IndexedString, by the way).

So in practice ItemRepositoryReferenceCounting's inc(), dec() and
setIndex() implement the move operations optimally => remove the more
complex moveIndex() helper function.

a17d3f65 similarly removed
IndexedString's move operations.