[clang] [clang] Implement P2582R1: CTAD from inherited constructors (PR #98788)

[Haojian Wu via cfe-commits]

================
@@ -1957,17 +1957,26 @@ class CXXDeductionGuideDecl : public FunctionDecl {
                         ExplicitSpecifier ES,
                         const DeclarationNameInfo &NameInfo, QualType T,
                         TypeSourceInfo *TInfo, SourceLocation EndLocation,
-                        CXXConstructorDecl *Ctor, DeductionCandidate Kind)
+                        CXXConstructorDecl *Ctor, DeductionCandidate Kind,
+                        CXXDeductionGuideDecl *GeneratedFrom,
+                        bool IsGeneratedFromInheritedConstructor)
       : FunctionDecl(CXXDeductionGuide, C, DC, StartLoc, NameInfo, T, TInfo,
                      SC_None, false, false, ConstexprSpecKind::Unspecified),
-        Ctor(Ctor), ExplicitSpec(ES) {
+        Ctor(Ctor), ExplicitSpec(ES),
+        SourceDeductionGuide(GeneratedFrom,
+                             IsGeneratedFromInheritedConstructor) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
     setDeductionCandidateKind(Kind);
   }
 
   CXXConstructorDecl *Ctor;
   ExplicitSpecifier ExplicitSpec;
+  // The deduction guide, if any, that this deduction guide was generated from,
+  // in the case of alias template deduction or CTAD from inherited
+  // constructors. The bool member indicates whether this deduction guide is
+  // generated from an inherited constructor.
----------------
hokein wrote:

I'd suggest using an enum type for the the source kind of the deduction, rather than using a `bool`.

```
enum SourceKind : bool {
  Alias,
  InheritedConstructor,
};
```

This also allows us to have a generic getter API `getSourceKind()`/`setSourceKind()`. 

https://github.com/llvm/llvm-project/pull/98788