[clang] [Clang] Implement P2718R0 "Lifetime extension in range-based for loops" (PR #76361)

[via cfe-commits]

cor3ntin wrote:

> Should we also dump the sub-expr which in `CXXDefaultArgExpr`: for example: Consider:
> 
> ```
> struct A {
>   A();
>   ~A();
> };
> 
> typedef int vec3[3];
> vec3 &f(const A &a = A{});
> 
> void foo() {
>   for (auto e : f()) {}
> }
> ```
> 
> Current AST Dump:
> 
> ```
> FunctionDecl 0x126813170 <line:85:1, line:87:1> line:85:6 foo 'void ()'
>   `-CompoundStmt 0x126814720 <col:12, line:87:1>
>     `-CXXForRangeStmt 0x1268146b0 <line:86:3, col:23>
>       |-<<<NULL>>>
>       |-DeclStmt 0x1268137b8 <col:17>
>       | `-VarDecl 0x126813548 <col:17, col:19> col:17 implicit used __range1 'vec3 &' cinit
>       |   `-ExprWithCleanups 0x126813730 <col:17, col:19> 'vec3':'int[3]' lvalue
>       |     `-CallExpr 0x126813318 <col:17, col:19> 'vec3':'int[3]' lvalue
>       |       |-ImplicitCastExpr 0x126813300 <col:17> 'vec3 &(*)(const A &)' <FunctionToPointerDecay>
>       |       | `-DeclRefExpr 0x126813278 <col:17> 'vec3 &(const A &)' lvalue Function 0x126813030 'f' 'vec3 &(const A &)'
>       |       `-CXXDefaultArgExpr 0x1268133f0 <<invalid sloc>> 'const A' lvalue
>       |-DeclStmt 0x126814448 <col:15>
>       | `-VarDecl 0x126813880 <col:15> col:15 implicit used __begin1 'int *' cinit
>  .......
> ```
> 
> We can't see directly on the AST whether the lifetime is extended or not. The sub-expr which in `CXXDefaultArgExpr` is:
> 
> ```
> ExprWithCleanups 0x1398c55d8 'const A':'const struct A' lvalue
> `-MaterializeTemporaryExpr 0x1398c55c0 'const A':'const struct A' lvalue extended by Var 0x1398c5748 '__range1' 'vec3 &'
>   `-ImplicitCastExpr 0x1398c55a8 'const A':'const struct A' <NoOp>
>     `-CXXBindTemporaryExpr 0x1398c5588 'A':'struct A' (CXXTemporary 0x1398c5588)
>       `-CXXTemporaryObjectExpr 0x1398c5550 'A':'struct A' 'void (void)'
> ```

I think this makes sense, but it can be a separate patch, unless it helps for testing.


FYI, we prefer merge commits instead of rebase + force push because Github reviews tend to loose context when doing force pushes / rebases in PRs

https://github.com/llvm/llvm-project/pull/76361