[Clang] [NFC] Prevent null pointer dereference in Sema::InstantiateFunctionDefinition (#89801)

In the lambda function within
clang::Sema::InstantiateFunctionDefinition, the return value of a
function that may return null is now checked before dereferencing to
avoid potential null pointer dereference issues which can lead to
crashes or undefined behavior in the program.

Author: smanna12

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -5184,6 +5184,7 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
 
     ParmVarDecl *Parm = Function->getParamDecl(0);
     TypeSourceInfo *NewParmSI = IR.TransformType(Parm->getTypeSourceInfo());
+    assert(NewParmSI && "Type transformation failed.");
     Parm->setType(NewParmSI->getType());
     Parm->setTypeSourceInfo(NewParmSI);
   };