[Sema] Use a TreeTransform to extract deduction guide parameter types

Previously, we just canonicalized the type, but this lead to crashes with
parameter types that referred to ParmVarDecls of the constructor. There may be
more cases that this TreeTransform needs to handle though, such as a constructor
parameter type referring to a member in an unevaluated context. Canonicalization
doesn't address these cases either though, so we can address them as-needed in
follow-up commits.

rdar://41330135

Differential revision: https://reviews.llvm.org/D49439

llvm-svn: 338165

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