meson: Cache os.path.realpath in CLikeCompilerArgs

Profiling showed that we were spending 25s inside os.path.realpath()
on Windows while generating compile lines for build.ninja, inside
NinjaBackend.generate()

The real path for these will not (should not) change during a single
meson invocation, so cache all these.

Brings build.ninja generation from 73s to 47s on my machine.

mesonbuild/compilers/mixins/clike.py

@@ -101,7 +101,7 @@ class CLikeCompilerArgs(arglist.CompilerArgs):
         # Remove system/default include paths added with -isystem
         default_dirs = self.compiler.get_default_include_dirs()
         if default_dirs:
-            real_default_dirs = [os.path.realpath(i) for i in default_dirs]
+            real_default_dirs = [self._cached_realpath(i) for i in default_dirs]
             bad_idx_list = []  # type: T.List[int]
             for i, each in enumerate(new):
                 if not each.startswith('-isystem'):
@@ -110,16 +110,21 @@ class CLikeCompilerArgs(arglist.CompilerArgs):
                 # Remove the -isystem and the path if the path is a default path
                 if (each == '-isystem' and
                         i < (len(new) - 1) and
-                        os.path.realpath(new[i + 1]) in real_default_dirs):
+                        self._cached_realpath(new[i + 1]) in real_default_dirs):
                     bad_idx_list += [i, i + 1]
-                elif each.startswith('-isystem=') and os.path.realpath(each[9:]) in real_default_dirs:
+                elif each.startswith('-isystem=') and self._cached_realpath(each[9:]) in real_default_dirs:
                     bad_idx_list += [i]
-                elif os.path.realpath(each[8:]) in real_default_dirs:
+                elif self._cached_realpath(each[8:]) in real_default_dirs:
                     bad_idx_list += [i]
             for i in reversed(bad_idx_list):
                 new.pop(i)
         return self.compiler.unix_args_to_native(new._container)
 
+    @staticmethod
+    @functools.lru_cache(maxsize=None)
+    def _cached_realpath(arg: str) -> str:
+        return os.path.realpath(arg)
+
     def __repr__(self) -> str:
         self.flush_pre_post()
         return f'CLikeCompilerArgs({self.compiler!r}, {self._container!r})'