[RFC][libc++][benchmark] Enable benchmark optimizations.

[mordante]

Before we moved our benchmark suite from a stand-alone suite to the current lit based suite the default was to run benchmarks optimized. With the change the default is to run not optimized. I've been bitten several times by this issue and need to spend time to look how to run these test optimized.

I feel this is a regression and also makes it harder for other people to do the right thing. I feel the default should be to do the right thing. I agree this somewhat differs from the idea that the test suite should be run with the same flags. Still I feel having the right default makes it easier to onboard in the project and not to remember these oddities.

This patch adds a new default optimization (bikeshedding is welcome) that keeps the main tests as if specified optimization=none and the benchmarks as if specified optimization=speed.
The other values keep their exisiting behaviour. (When the test are optimized for speed or size the benchmarks will inherit this flag without any changes.) The benchmarks can still be not opimized by using optimization=none.

This patch will need more cleanups and documentation.

[ldionne]

I feel like this is clearly documented in https://libcxx.llvm.org/TestingLibcxx.html#benchmarks, but I guess making the default closer to what we usually want is not a bad thing. Overall I think this patch makes sense, please ping me again when it's ready for a final review (it seems like not much is missing).

[ldionne]

Suggested change

	default="default", # defaults, test -> none, benchmarks -> speed
	help="The optimization level to use when compiling the test suite. 'default' is to enable no optimizations for tests and to optimize benchmarks for speed.",
	default="default", # defaults, test -> none, benchmarks -> speed