[FEA]: Remove unsupported configurations from libcu++

[miscco]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this request and that I agree to the Code of Conduct

Component

libcu++

Is your feature request related to a problem? Please describe.

Due to historical reasons, we have a ton of cruft in the code base. This comes from libc++ supporting compilation modes / platform that we do not support. This includes (not exhausting):

• Conditional noexcept support.
  • We do not support any standard prior to C++11 so noexcept is supported by all compilers we target
• C++03 support
  • There is a significant amount of churn around this, as it prevents usage of usingdeclarations and variadics
• Unsupported platforms such as SUN / IBM

Describe the solution you'd like

We should identify unneeded configurations and remove them to reduce code complexity

Describe alternatives you've considered

We can leave everything as is, as long as we remember why we are doing it.

Additional context

No response