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

This is a cli tool to that tests the conformance of LLVM's mustache
implementation against the public Mustache spec, hosted at
https://github.com/mustache/spec. This is a revised version of the
patches in #111487.

Co-authored-by: Peter Chou <peter.chou@mail.utoronto.ca>

llvm/docs/CommandGuide/index.rst

@@ -87,6 +87,7 @@ Developer Tools
    llvm-exegesis
    llvm-ifs
    llvm-locstats
+   llvm-test-mustache-spec
    llvm-pdbutil
    llvm-profgen
    llvm-tli-checker

llvm/docs/CommandGuide/llvm-test-mustache-spec.rst

@@ -0,0 +1,37 @@
+llvm-test-mustache-spec - LLVM tool to test Mustache library compliance
+=======================================================================
+
+.. program:: llvm-test-mustache-spec
+
+SYNOPSIS
+--------
+
+:program:`llvm-test-mustache-spec` [*inputs...*]
+
+Description
+-----------
+
+``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.
+
+The tool prints the number of test failures and successes in each of the test
+files to standard output.
+
+EXAMPLE
+-------
+
+.. code-block:: console
+
+   $ llvm-test-mustache-spec path/to/specs/\*.json
+