Partially inline basic_string::operator=(const basic_string&)

Summary:
This change partially inlines operator=(const basic_string&) where both the input and current instance are short strings, making the assignment a fixed length inlined memcpy.

Assignments where either of the strings are long are delegate to __assign_no_alias<__is_short>(), which is templated for the long / short branch already observed in the caller.

Stable:
```
--------------------------------------------------------------------------------
Benchmark                                     Time             CPU   Iterations
--------------------------------------------------------------------------------
BM_StringAssignStr_Empty_Opaque            2.65 ns         2.66 ns    263745536
BM_StringAssignStr_Empty_Transparent       2.95 ns         2.96 ns    236494848
BM_StringAssignStr_Small_Opaque            2.93 ns         2.94 ns    237301760
BM_StringAssignStr_Small_Transparent       2.69 ns         2.69 ns    265809920
BM_StringAssignStr_Large_Opaque            19.6 ns         19.6 ns     35573760
BM_StringAssignStr_Large_Transparent       19.1 ns         19.1 ns     36716544
BM_StringAssignStr_Huge_Opaque             1901 ns         1901 ns       364544
BM_StringAssignStr_Huge_Transparent        1889 ns         1889 ns       360448
```

Unstable
```
--------------------------------------------------------------------------------
Benchmark                                     Time             CPU   Iterations
--------------------------------------------------------------------------------
BM_StringAssignStr_Empty_Opaque            1.29 ns         1.29 ns    540454912
BM_StringAssignStr_Empty_Transparent       1.11 ns         1.12 ns    628482048
BM_StringAssignStr_Small_Opaque            1.29 ns         1.29 ns    541216768
BM_StringAssignStr_Small_Transparent       1.11 ns         1.11 ns    629469184
BM_StringAssignStr_Large_Opaque            15.6 ns         15.6 ns     44945408
BM_StringAssignStr_Large_Transparent       14.9 ns         14.9 ns     46764032
BM_StringAssignStr_Huge_Opaque             1713 ns         1713 ns       401408
BM_StringAssignStr_Huge_Transparent        1704 ns         1704 ns       397312

```

Subscribers: libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75211