Adapt per feature ABI breakage for this change

• fix ifdef for __init_long

• Remove old defines

• Relocate define in __config

I added alternaive DIFF in https://reviews.llvm.org/D69061

Thanks for the elaborate comments Louis!

1. Updating D68617: partially inline copy constructor basic_string(const basic_string&[, allocator]) #
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Updated with ABI in unstable

This change highlights ABI issues (init_long is not a forwards compatible change / technically breaks ABI)

Re split for the alloc case: that may be a good option for a next change, I wanted to keep this change small in scope, and for std::allocator no extra cost / code emitted

Yeah, it's pretty bad though if allocators have throwing move operators, but until c++14 it's 'technically allowed' and something that the std handle gracefully. I was not that alarmed tho, such should be rare, and deserving a halt and catch fire. :)

libc++.so code for std::string ctor (demangled / mixed source)

Here's the godbolt link I am looking at with your code:

1. I updated the title

• Added inline

• Merge branch 'master' of https://github.com/llvm/llvm-project into add-benchmarks

• Optimize operator=(const basic_string&) for tail call.

Resolved comments