[llvm] [UpdateTestChecks] Escape special characters more consistently (PR #226203)

[Nikita Popov via llvm-commits]

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@@ -0,0 +1,3 @@
+## Check escaping of [[, which can occur in global initializers.
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nikic wrote:

Ooops, fixed.

I also had to move the escaping even further up because it interfered with `--replace-value-regex`, as the update_cc_test_checks tests caught.

https://github.com/llvm/llvm-project/pull/226203