[clang] fix broken canonicalization of DeducedTemplateSpecializationType

This reverts the functional elements of commit 3e78fa86
and redoes it, by fixing the true root cause of #61317.

A TemplateName can be non-canonical; profiling it based on the canonical
name would result in inconsistent preservation of as-written information
in the AST.

The true problem in #61317 is that we would not consider the methods
with requirements expression which contain DTSTs as the same
in relation to merging of declarations when importing modules.

The expressions would never match because they contained DTSTs pointing
to different redeclarations of the same class template, but since
canonicalization for them was broken, their canonical types would not
match either.

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No changelog entry because #61317 was already claimed as fixed in
previous release.