fix ninja backend rules containing internal enum reprs

Partially reverts commit 1624354 which
moved a bunch of stuff from strings to enums. The issue here is that
Compiler.mode is not just, or primarily, something we compare, but is
instead written in as e.g. `rule c_{compiler.mode}` to build.ninja, so
this identifier needs to be a string.

Ultimately, the issue is that the commit tried to rewrite a bunch of
things called "mode" that had a couple of TODOs saying to use enums...
but it rewrote everything called "mode" regardless of whether it was a
function kwarg or a compiler property, even though the TODO only applied
to one of them.

mesonbuild/compilers/compilers.py

@@ -527,7 +527,7 @@ class Compiler(HoldableObject, metaclass=abc.ABCMeta):
     language: str
     id: str
     warn_args: T.Dict[str, T.List[str]]
-    mode = CompileCheckMode.COMPILE
+    mode = 'COMPILER'
 
     def __init__(self, ccache: T.List[str], exelist: T.List[str], version: str,
                  for_machine: MachineChoice, info: 'MachineInfo',

mesonbuild/compilers/mixins/clike.py

@@ -1359,14 +1359,14 @@ def get_assert_args(self, disable: bool) -> T.List[str]:
     @functools.lru_cache(maxsize=None)
     def can_compile(self, src: 'mesonlib.FileOrString') -> bool:
         # Files we preprocess can be anything, e.g. .in
-        if self.mode == CompileCheckMode.PREPROCESS:
+        if self.mode == 'PREPROCESSOR':
             return True
         return super().can_compile(src)
 
     def get_preprocessor(self) -> Compiler:
         if not self.preprocessor:
             self.preprocessor = copy.copy(self)
             self.preprocessor.exelist = self.exelist + self.get_preprocess_to_file_args()
-            self.preprocessor.mode = CompileCheckMode.PREPROCESS
+            self.preprocessor.mode = 'PREPROCESSOR'
             self.modes.append(self.preprocessor)
         return self.preprocessor

mesonbuild/compilers/mixins/visualstudio.py

@@ -182,7 +182,7 @@ def sanitizer_compile_args(self, value: str) -> T.List[str]:
         return ['/fsanitize=address']
 
     def get_output_args(self, outputname: str) -> T.List[str]:
-        if self.mode == CompileCheckMode.PREPROCESS:
+        if self.mode == 'PREPROCESSOR':
             return ['/Fi' + outputname]
         if outputname.endswith('.exe'):
             return ['/Fe' + outputname]