[libc++] Allow detecting whether the executor supports Bash

A few tests in the test suite require support for Bash. For example,
tests that run a program and send data through stdin to it require some
way of piping the data in, and we use a Bash script for that.

However, some executors (e.g. an embedded systems simulator) do not
support Bash, so these tests will fail. This commit adds a Lit feature
that tries to detect whether Bash is available through conventional
means, and disables the tests that require it otherwise.

Differential Revision: https://reviews.llvm.org/D114612

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