[clang][sema] Generate builtin operator overloads for (volatile) _Atomic types

We observed a failed assert in overloaded compound-assignment operator resolution:

```
Assertion failed: (Result.isInvalid() && "C++ binary operator overloading is missing candidates!"), function CreateOverloadedBinOp, file SemaOverload.cpp, line 13944.
...
frame #4: clang` clang::Sema::CreateOverloadedBinOp(..., Opc=BO_OrAssign, ..., PerformADL=true, AllowRewrittenCandidates=false, ...) at SemaOverload.cpp:13943
frame #5: clang` BuildOverloadedBinOp(..., Opc=BO_OrAssign, ...) at SemaExpr.cpp:15228
frame #6: clang` clang::Sema::BuildBinOp(..., Opc=BO_OrAssign, ...) at SemaExpr.cpp:15330
frame #7: clang` clang::Sema::ActOnBinOp(..., Kind=pipeequal, ...) at SemaExpr.cpp:15187
frame #8: clang` clang::Parser::ParseRHSOfBinaryExpression(..., MinPrec=Assignment) at ParseExpr.cpp:629
frame #9: clang` clang::Parser::ParseAssignmentExpression(..., isTypeCast=NotTypeCast) at ParseExpr.cpp:176
frame #10: clang` clang::Parser::ParseExpression(... isTypeCast=NotTypeCast) at ParseExpr.cpp:124
frame #11: clang` clang::Parser::ParseExprStatement(...) at ParseStmt.cpp:464
```

A simple reproducer is:

```
_Atomic unsigned an_atomic_uint;

enum { an_enum_value = 1 };

void enum1() { an_atomic_uint += an_enum_value; }
```

This patch fixes the issue by generating builtin operator overloads for (volatile) _Atomic types.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D125349