BTreeMap: improve gdb introspection of BTreeMap with ZST keys or values #77932
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I accidentally pushed an earlier revision in #77788: it changes the index of tuples for BTreeSet from ""[{}]".format(i) to "key{}".format(i). Which doesn't seem to make the slightest difference on my linux box nor on CI. In fact, gdb doesn't make any distinction between "key{}" and "val{}" for a BTreeMap either, leading to confusing output if you test more. But easy to improve.
r? @Mark-Simulacrum