Bump bootstrap compiler version to 1.55#87669

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fee1-dead wants to merge 2 commits intorust-lang:masterfrom

fee1-dead-contrib:bump-bootstrap

Commits on Jul 30, 2021

bump bootstrap compiler to 1.55
pietroalbini
committed

Commits on Jul 31, 2021

Readd const_fn_union feature
fee1-dead
committed