THIR patterns: Always use type `str` for string-constant-value nodes
Historically, constants and literals of type `&str` have been represented in THIR patterns as `PatKind::Const` nodes with type `&str`.
That's fine for stable Rust, but `feature(deref_patterns)` also created a need to have string literal patterns of type `str` in some cases, which resulted in a number of additional special cases and inconsistencies in typechecking and in HIR-to-THIR-to-MIR lowering of patterns.
We can avoid several of those special cases by having THIR treat string-constant-values as fundamentally being of type `str`, and then using `PatKind::Deref` to represent the additional `&` layer in the common case where it is needed. This allows bare `str` patterns to require very little special treatment.
Existing tests should already do a good job of demonstrating that this implementation change does not affect the stable language.
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