[llvm] [llvm] Add a tool to check mustache compliance against the public spec (PR #142813)

[Nikita Popov via llvm-commits]

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+llvm-test-mustache-spec - LLVM tool to test Mustache Compliance Library
+=========================================================================
+
+llvm-test-mustache-spec tests the mustache spec conformance of the LLVM
+mustache library. The spec can be found here: https://github.com/mustache/spec
+
+To test against the spec, simply download the spec and pass the test JSON files
+to the driver. Each spec file should have a list of tests for compliance with
+the spec. These are loaded as test cases, and rendered with our Mustache
+implementation, which is then compared against the expected output from the
+spec.
+
+The current implementation only supports non-optional parts of the spec, so
+we do not expect any of the dynamic-names, inheritance, or lambda tests to
+pass. Additionally, Triple Mustache is not supported. Unsupported tests are
+marked as XFail and are removed from the XFail list as they are fixed.
+
+    $ llvm-test-mustache-spec path/to/test/file.json path/to/test/file2.json ...
----------------
nikic wrote:

```suggestion
    $ llvm-test-mustache-spec path/to/specs/*.json
```
Maybe?

https://github.com/llvm/llvm-project/pull/142813