DebugInfo: Make DICompositeTypes distinct most of the time

Since elements of most kinds of DICompositeType have back references,
most are involved in uniquing cycles.  Except via the ODR 'identifier:'
field, which doesn't care about the storage type (see r266549),
they have no hope of being uniqued.

Distinct nodes are far more efficient, so use them for most kinds of
DICompositeType definitions (i.e., when DIType::isForwardDecl is false).
The exceptions:

  - DW_TAG_array_type, since their elements never have back-references
    and they never have ODR 'identifier:' fields;

  - DW_TAG_enumeration_type when there is no ODR 'identifier:' field,
    since their elements usually don't have back-references.

This breaks the last major uniquing cycle I'm aware of in the debug info
graph.  The impact won't be enormous for C++ because references to
ODR-uniqued nodes still use string-based DITypeRefs; but this should
prevent a regression in C++ when we drop the string-based references.

This wouldn't have been reasonable until r266549, when composite types
stopped relying on being uniqued by structural equivalence to prevent
blow-ups at LTO time.

llvm-svn: 266556