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

Patch by Martijn Vels (mvels@google.com)
Reviewed as https://reviews.llvm.org/D68276

This is a non trivial win for externally templated assignment operator.

x86 without tail call (current libc++)

0000000000000000 <std::string::operator=(std::string const&)>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   53                      push   %rbx
   5:   50                      push   %rax
   6:   48 89 fb                mov    %rdi,%rbx
   9:   48 39 f7                cmp    %rsi,%rdi
   c:   74 17                   je     25 <std::string::operator=(std::string const&)+0x25>
   e:   0f b6 56 17             movzbl 0x17(%rsi),%edx
  12:   84 d2                   test   %dl,%dl
  14:   79 07                   jns    1d <std::string::operator=(std::string const&)+0x1d>
  16:   48 8b 56 08             mov    0x8(%rsi),%rdx
  1a:   48 8b 36                mov    (%rsi),%rsi
  1d:   48 89 df                mov    %rbx,%rdi
  20:   e8 00 00 00 00          callq  25 <std::string::operator=(std::string const&)+0x25>
  25:   48 89 d8                mov    %rbx,%rax
  28:   48 83 c4 08             add    $0x8,%rsp
  2c:   5b                      pop    %rbx
  2d:   5d                      pop    %rbp
  2e:   c3                      retq

After:

0000000000000000 <std::string::operator=(std::string const&)>:
   0:   48 39 f7                cmp    %rsi,%rdi
   3:   74 14                   je     19 <std::string::operator=(std::string const&)+0x19>
   5:   0f b6 56 17             movzbl 0x17(%rsi),%edx
   9:   84 d2                   test   %dl,%dl
   b:   79 07                   jns    14 <std::string::operator=(std::string const&)+0x14>
   d:   48 8b 56 08             mov    0x8(%rsi),%rdx
  11:   48 8b 36                mov    (%rsi),%rsi
  14:   e9 00 00 00 00          jmpq   19 <std::string::operator=(std::string const&)+0x19>
  19:   48 89 f8                mov    %rdi,%rax
  1c:   c3                      retq

Benchmark (pending per https://reviews.llvm.org/D67667)

```
BM_StringAssignStr_Empty_Opaque                     6.23ns ± 0%             5.19ns ± 0%  -16.70%          (p=0.016 n=5+4)
BM_StringAssignStr_Empty_Transparent                5.86ns ± 0%             5.14ns ± 0%  -12.24%          (p=0.008 n=5+5)
BM_StringAssignStr_Small_Opaque                     8.79ns ± 1%             7.69ns ± 0%  -12.53%          (p=0.008 n=5+5)
BM_StringAssignStr_Small_Transparent                9.44ns ± 0%             8.00ns ± 0%  -15.26%          (p=0.008 n=5+5)
BM_StringAssignStr_Large_Opaque                     25.2ns ± 0%             24.3ns ± 0%   -3.50%          (p=0.008 n=5+5)
BM_StringAssignStr_Large_Transparent                23.6ns ± 0%             22.5ns ± 0%   -4.76%          (p=0.008 n=5+5)
BM_StringAssignStr_Huge_Opaque                       319ns ± 5%              317ns ± 5%     ~             (p=0.690 n=5+5)
BM_StringAssignStr_Huge_Transparent                  319ns ± 5%              317ns ± 5%     ~             (p=0.421 n=5+5)
BM_StringAssignAsciiz_Empty_Opaque                  7.41ns ± 0%             7.77ns ± 0%   +4.89%          (p=0.008 n=5+5)
BM_StringAssignAsciiz_Empty_Transparent             7.54ns ± 3%             7.30ns ± 0%   -3.24%          (p=0.008 n=5+5)
BM_StringAssignAsciiz_Small_Opaque                  9.87ns ± 0%            10.24ns ± 1%   +3.76%          (p=0.008 n=5+5)
BM_StringAssignAsciiz_Small_Transparent             10.4ns ± 1%              9.8ns ± 2%   -5.78%          (p=0.008 n=5+5)
BM_StringAssignAsciiz_Large_Opaque                  30.1ns ± 0%             30.1ns ± 0%     ~             (p=0.167 n=5+5)
BM_StringAssignAsciiz_Large_Transparent             27.1ns ± 0%             27.4ns ± 0%   +0.92%          (p=0.016 n=4+5)
BM_StringAssignAsciiz_Huge_Opaque                    383ns ± 4%              382ns ± 4%     ~             (p=0.548 n=5+5)
BM_StringAssignAsciiz_Huge_Transparent               375ns ± 0%              380ns ± 0%   +1.37%          (p=0.029 n=4+4)
BM_StringAssignAsciizMix_Opaque                     14.0ns ± 0%             14.0ns ± 0%     ~             (p=0.881 n=5+5)
BM_StringAssignAsciizMix_Transparent                13.7ns ± 1%             13.8ns ± 0%     ~             (p=0.056 n=5+5)
```

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374137 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -2260,7 +2260,7 @@ basic_string<_CharT, _Traits, _Allocator>::operator=(const basic_string& __str)
     if (this != &__str)
     {
         __copy_assign_alloc(__str);
-        assign(__str.data(), __str.size());
+        return assign(__str.data(), __str.size());
     }
     return *this;
 }