mitigated -Wunused-function Clang warnings with code added by matchcompiler (danmar#7810)

```
/home/user/CLionProjects/cppcheck/cmake-build-relwithdebinfo-clang/lib/build/mc_checkinternal.cpp:7:20: warning: unused function 'match1' [-Wunused-function]
    7 | static inline bool match1(const Token* tok) {
      |                    ^~~~~~
```

Author: firewave

tools/matchcompiler.py

@@ -206,7 +206,7 @@ def _compilePattern(self, pattern, nr, varid,
                 arg2 = ', const int varid'
 
             ret = '// pattern: ' + pattern + '\n'
-            ret += 'static inline bool match' + \
+            ret += 'MAYBE_UNUSED static inline bool match' + \
                 str(nr) + '(' + tokenType + '* tok' + arg2 + ') {\n'
             returnStatement = 'return false;\n'
 
@@ -290,7 +290,7 @@ def _compileFindPattern(self, pattern, findmatchnr, endToken, varId):
             more_args += ', int varid'
 
         ret = '// pattern: ' + pattern + '\n'
-        ret += 'template<class T> static inline T * findmatch' + \
+        ret += 'template<class T> MAYBE_UNUSED static inline T * findmatch' + \
             str(findmatchnr) + '(T * start_tok' + more_args + ') {\n'
         ret += '    for (; start_tok' + endCondition + \
             '; start_tok = start_tok->next()) {\n'
@@ -373,7 +373,7 @@ def _compileVerifyTokenMatch(
         if varId:
             more_args = ', const int varid'
 
-        ret = 'static inline bool match_verify' + \
+        ret = 'MAYBE_UNUSED static inline bool match_verify' + \
             str(verifyNumber) + '(const Token *tok' + more_args + ') {\n'
 
         origMatchName = 'Match'
@@ -509,7 +509,7 @@ def _compileVerifyTokenFindMatch(
         if varId:
             more_args += ', const int varid'
 
-        ret = 'template < class T > static inline T * findmatch_verify' + \
+        ret = 'template < class T > MAYBE_UNUSED static inline T * findmatch_verify' + \
             str(verifyNumber) + '(T * tok' + more_args + ') {\n'
 
         origFindMatchName = 'findmatch'
@@ -721,13 +721,29 @@ def convertFile(self, srcname, destname, line_directive):
         if len(self._rawMatchFunctions):
             header += '#include "errorlogger.h"\n'
             header += '#include "token.h"\n'
+            header += '#if defined(__clang__)\n'
+            header += '#include "config.h"\n'
+            header += '#define MAYBE_UNUSED [[maybe_unused]]\n'  # this attribute is also available in earlier standards
+            header += 'SUPPRESS_WARNING_CLANG_PUSH("-Wc++17-attribute-extensions")\n'  # suppress waning about using above attribute in earlier standard
+            header += '#else\n'
+            header += '#define MAYBE_UNUSED\n'
+            header += '#endif\n'
+
+        footer = ''
+        if len(self._rawMatchFunctions):
+            footer += '#if defined(__clang__)\n'
+            footer += 'SUPPRESS_WARNING_CLANG_POP\n'
+            footer += '#endif\n'
+            footer += '#undef MAYBE_UNUSED\n'
 
         with io.open(destname, 'wt', encoding="utf-8") as fout:
             if modified or len(self._rawMatchFunctions):
                 fout.write(header)
                 fout.write(strFunctions)
             fout.write(lineno)
             fout.write(code)
+            if modified or len(self._rawMatchFunctions):
+                fout.write(footer)
 
 
 def main():