improve the AST for the synthesized deduction guide

[hokein]

Given the following CTAD code:

template <typename T>
struct C {
    template <typename U>
    C(U);
};
C c(1);

The synthesized deduction guide from the constructor is:

|-FunctionTemplateDecl  implicit <deduction guide for C>
| |-TemplateTypeParmDecl <line:1:11, col:20> col:20 typename depth 0 index 0 T
| |-TemplateTypeParmDecl <line:3:15, col:24> col:24 typename depth 0 index 1 U
| `-CXXDeductionGuideDecl <line:4:5, col:8> col:5 implicit <deduction guide for C> 'auto (type-parameter-0-1) -> C<T>'
|   `-ParmVarDecl 0x555d18fe4638 <col:7> col:8 'type-parameter-0-1'

Note that the function parameter type is canonicalized to type-parameter-0-1, this is suboptimal. I think it is better to show a normal U.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

Given the following CTAD code:

template <typename T>
struct C {
    template <typename U>
    C(U);
};
C c(1);

The synthesized deduction guide from the constructor is:

|-FunctionTemplateDecl  implicit <deduction guide for C>
| |-TemplateTypeParmDecl <line:1:11, col:20> col:20 typename depth 0 index 0 T
| |-TemplateTypeParmDecl <line:3:15, col:24> col:24 typename depth 0 index 1 U
| `-CXXDeductionGuideDecl <line:4:5, col:8> col:5 implicit <deduction guide for C> 'auto (type-parameter-0-1) -> C<T>'
|   `-ParmVarDecl 0x555d18fe4638 <col:7> col:8 'type-parameter-0-1'

Note that the function parameter type is canonicalized to type-parameter-0-1, this is suboptimal. I think it is better to show a normal U.