make use of C++11's auto type deduction for temporary variables (#23327)

This is just one of those tiny steps towards the goal of an "optimized"
C and C++ codegen I raised elsewhere before - what does me babbling
"optimized" mainly entail?

(not mutually-exclusive ascertainment proposals following:)

- less and simplified resulting code: easier to pick up/grasp for the
C/C++ compiler for to do its own optimization heuristics, less parsing
effort for us mere humans trying to debug, especially in the case of
interop
- build time reduction: less code emission I/O, runtime string
formatting for output...
- easier access for fresh contributors and better maintainability
- interop improvements
- further runtime optimizations

I am eagerly looking forward to the results of the LLVM-based
undertakings, but I also think we can do a bit better (as outlined
above) with our current C/C++ backends till those come to fruition.

**Long story short**: this PR here focuses on the C++ backend,
augmenting the current codegen method of establishing "temporary"
variables by using C++11's auto type deduction. The reasons for adopting
an "Almost Always Auto" style have been collected [here
](https://herbsutter.com/2013/08/12/gotw-94-solution-aaa-style-almost-always-auto/)
for the C++ world. For those hopping between C++'s and Nim's realms,
this change also results in a bit less code and less work for the
codegen part (no redundant `getTypeDesc`s): no need to tell the C++
compiler the type it already knows of (in most cases).

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