[utils] Add script to generate elaborated IR and assembly tests (#89026)

Generally, IR and assembly test files benefit from being cleaned to
remove unnecessary details. However, for tests requiring

[utils] Add script to generate elaborated IR and assembly tests (#89026)

Generally, IR and assembly test files benefit from being cleaned to
remove unnecessary details. However, for tests requiring elaborate
IR or assembly files where cleanup is less practical (e.g., large amount
of debug information output from Clang), the current practice is to
include the C/C++ source file and the generation instructions as
comments.

This is inconvenient when regeneration is needed. This patch adds
`llvm/utils/update_test_body.py` to allow easier regeneration.

`ld.lld --debug-names` tests (#86508) utilize this script for
Clang-generated assembly tests.

Note: `-o pipefail` is standard (since
https://www.austingroupbugs.net/view.php?id=789) but not supported by
dash.

Link:
https://discourse.llvm.org/t/utility-to-generate-elaborated-assembly-ir-tests/78408

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