Incorrect depth for SubstTemplateTypeParmType for alias template substitution.

[mizvekov]

Our substitution of alias templates is leaving behind a SubstTemplateTypeParmType pointing to TemplateTypeParmType with incorrect depth.

Example: (https://godbolt.org/z/r3GPj7GGn)

template<class T> struct C {
  template<class U> using type1 = U (T);
};
using type2 = typename C<int>::type1<void>;

AST dump of type2:

`-TypeAliasDecl 0x56240cfed3e8 <line:4:1, col:42> col:7 type2 'typename C<int>::type1<void>':'void (int)'
  `-ElaboratedType 0x56240cfed340 'typename C<int>::type1<void>' sugar
    `-TemplateSpecializationType 0x56240cfce350 'type1<void>' sugar alias type1
      |-TemplateArgument type 'void'
      | `-BuiltinType 0x56240cf831b0 'void'
      `-FunctionProtoType 0x56240cfce2f0 'void (int)' cdecl
        |-SubstTemplateTypeParmType 0x56240cfce290 'void' sugar
        | |-TemplateTypeParmType 0x56240cfcdfa0 'U' dependent depth 0 index 0
        | | `-TemplateTypeParm 0x56240cfcdf50 'U'
        | `-BuiltinType 0x56240cf831b0 'void'
        `-SubstTemplateTypeParmType 0x56240cfcdfe0 'int' sugar
          |-TemplateTypeParmType 0x56240cfcd590 'T' dependent depth 0 index 0
          | `-TemplateTypeParm 0x56240cfcd510 'T'
          `-BuiltinType 0x56240cf83250 'int'

Ie notice in the AST both T and U are at the same depth, 0.

This happens because we substitute T early into the alias, which pulls U down one level, and then we substitute U.
This flattens everything down to ground level.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[mizvekov]

MR for fix: https://reviews.llvm.org/D127351