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