String de-duplication for localtime_r

[tiif]

Since this is already brought up in #3470 (comment), I will just open a new issue for this.

To resolve the FIXME here,

miri/src/shims/time.rs

Lines 198 to 199 in 052bdcb

	// FIXME: String de-duplication is needed so that we only allocate this string only once
	// even when there are multiple calls to this function.

we could do:

We should have allocate_bytes which allocates a byte slice, and that should use the cache. allocate_str can call that, and transmute the result to &str.

[tiif]

@rustbot label +C-enhancement