Split [class.temporary]

[class.temporary] talks about two very different kinds of temporaries: the most well-known ones, which are created by temporary materialization, and the much less well-known ones, which are created at the implementation's discretion to store function parameters and return values in registers.

It would be helpful to be able to cross-reference the latter specifically, for example in Example 2 to [dcl.contract.res]. We can't currently do that because we can't cross-reference a particular paragraph within a section.

My suggestion is that we create a new subsection [class.temporary.general] which contains current paragraph 1, followed by [class.temporary.materialization] which contains current paragraph 2, followed by [class.temporary.function] which contains current paragraph 3, and finally [class.temporary.lifetime] which contains all remaining paragraphs.

This is proposed to become part of #7399

Start by locating the source for [class.temporary] and read the proposal in #7399 for surrounding context. Split the section into the proposed general, materialization, function, and lifetime subsections, preserving the specified paragraph grouping; done means the relevant paragraphs can be cross-referenced individually.