[lit] Add a substitution for Clang Analyzer tests

[miyuki]

Summary

The substitution will reduce duplication in Clang test cases.

[george.karpenkov]

I'm not sure it makes sense to put it into the LLVM repo.
What about clang/test/lit.cfg.py? (or, if possible, inside clang/test/Analysis)

utils/lit/lit/llvm/config.py

def use_clang(self, required=True):
             ToolSubst('%clang_analyze_cc1', command='%clang_cc1', extra_args=['-analyze', '%analyze']),
             ToolSubst('%clang_cc1', command=self.config.clang, extra_args=['-cc1', '-internal-isystem', builtin_include_dir, '-nostdsysteminc']),
             ToolSubst('%clang_cpp', command=self.config.clang, extra_args=['--driver-mode=cpp']),
             ToolSubst('%clang_cl', command=self.config.clang, extra_args=['--driver-mode=cl']),
             ToolSubst('%clangxx', command=self.config.clang, extra_args=['--driver-mode=g++']),
             ]
         self.add_tool_substitutions(tool_substitutions)
 
+        # Diff command used by Clang Analyzer tests (when comparing .plist files
+        # with reference output)
+        self.config.substitutions.append(('%diff_plist',
+                                          'diff -u -w -I "<string>/" -I "<string>.:" -I "version" -'))
+
         self.config.substitutions.append(('%itanium_abi_triple',
                                           self.make_itanium_abi_triple(self.config.target_triple)))
         self.config.substitutions.append(('%ms_abi_triple',
                                           self.make_msabi_triple(self.config.target_triple)))
         self.config.substitutions.append(
             ('%resource_dir', builtin_include_dir))
 
         # The host triple might not be set, at least if we're compiling clang from