[libc++] Optimize / partially inline basic_string copy constructor

Splits copy constructor up inlining short initialization, outlining long
initialization into __init_long() which is the externally instantiated slow
path initialization.

Subsequently changing the copy ctor to be inlined (not externally instantiated)
provides significant speed ups for short string initialization.

Generated code given:

void StringCopyCtor(void* mem, const std::string& s) {
    std::string*p = new(mem) std::string{s};
}

asm:
        cmp     byte ptr [rsi + 23], 0
        js      .LBB0_2
        mov     rax, qword ptr [rsi + 16]
        mov     qword ptr [rdi + 16], rax
        movups  xmm0, xmmword ptr [rsi]
        movups  xmmword ptr [rdi], xmm0
        ret
.LBB0_2:
        jmp     std::basic_string::__init_long # TAILCALL

Benchmark:
BM_StringCopy_Empty                                           5.19ns ± 6%             1.50ns ± 8%  -71.02%        (p=0.000 n=10+10)
BM_StringCopy_Small                                           5.14ns ± 8%             1.53ns ± 7%  -70.17%        (p=0.000 n=10+10)
BM_StringCopy_Large                                           18.9ns ± 0%             19.3ns ± 0%   +1.92%        (p=0.000 n=10+10)
BM_StringCopy_Huge                                             309ns ± 1%              316ns ± 5%     ~            (p=0.633 n=8+10)

Patch from Martijn Vels (mvels@google.com)
Reviewed as D72160.