[clang-tidy][modernize-loop-convert] Make loop var type human readable

[zinovy.nis]

Summary

Without this patch

for(std::set<int>::iterator it = s.begin(); ...)

is converted into

for(std::_Tree_const_iterator<std::_Tree_val<std::_Tree_simple_types<int>>>::value_type i : s)

With this patch the output is as expected:

for(int i : s)

Related bug: https://bugs.llvm.org/show_bug.cgi?id=36688

[njames93]

I'm having trouble with the reproduction of this - https://godbolt.org/z/tsMfcj.
Aside from that this needs some test cases to demonstrate the patch is indeed working

[zinovy.nis]

In D80536#2055500, @njames93 wrote:

I'm having trouble with the reproduction of this - https://godbolt.org/z/tsMfcj.
Aside from that this needs some test cases to demonstrate the patch is indeed working

Thanks. Seems like the reproduction is c++-library dependent. Please look at https://godbolt.org/z/tbGG9b where I have manually "emulated" std::set. Or just change compiler to "clang-tidy (trunk) #1 with x86-64 clang (std::embed)" in your sample.
I get the same result (as for my custom "class set") for std::set on 'master' on Windows 10.

Aside from that this needs some test cases to demonstrate the patch is indeed working

IMHO, for testing purposes it's enough to apply my change to 'structures.h' (int -> value_type)

[zinovy.nis]

@njames93, @aaron.ballman do you have any other questions/comments?

[njames93]

LGTM

clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp

void LoopConvertCheck::doConversion(
   }
 
   // Now, we need to construct the new range expression.
   SourceRange ParenRange(Loop->getLParenLoc(), Loop->getRParenLoc());
 
   QualType Type = Context->getAutoDeductType();
   if (!Descriptor.ElemType.isNull() && Descriptor.ElemType->isFundamentalType())
     Type = Descriptor.ElemType.getUnqualifiedType();
+  Type = Type.getDesugaredType(*Context);
 
   // If the new variable name is from the aliased variable, then the reference
   // type for the new variable should only be used if the aliased variable was
   // declared as a reference.
   bool IsCheapToCopy =
       !Descriptor.ElemType.isNull() &&
       Descriptor.ElemType.isTriviallyCopyableType(*Context) &&
       // TypeInfo::Width is in bits.

clang-tools-extra/test/clang-tidy/checkers/Inputs/modernize-loop-convert/structures.h

struct S {
   const_iterator end() const;
   const_iterator cbegin() const;
   const_iterator cend() const;
   iterator begin();
   iterator end();
 };
 
 struct T {
+  typedef int value_type;
   struct iterator {
-    int& operator*();
-    const int& operator*()const;
+    value_type &operator*();
+    const value_type &operator*() const;
     iterator& operator ++();
     bool operator!=(const iterator &other);
-    void insert(int);
-    int X;
+    void insert(value_type);
+    value_type X;
   };
   iterator begin();
   iterator end();
 };
 
 struct U {
   struct iterator {
     Val& operator*();