interpreter: move backend type checking to interpreter

Of course, this checking isn't actually doing what it claims, as it
doesn't actually stop you from writing at target like:
```meson
build_target(
  'foo',
  'srcs/main.c',
  c_pch : 'src/pch.h',
)
```

Author: dcbaker

mesonbuild/backend/ninjabackend.py

@@ -3380,10 +3380,6 @@ def generate_pch(self, target: build.BuildTarget, header_deps=None):
             pch = target.pch[lang]
             if not pch:
                 continue
-            if not has_path_sep(pch[0]) or (pch[1] and has_path_sep(pch[1])):
-                msg = f'Precompiled header of {target.get_basename()!r} must not be in the same ' \
-                      'directory as source, please put it in a subdirectory.'
-                raise InvalidArguments(msg)
             compiler: Compiler = target.compilers[lang]
             if compiler.get_argument_syntax() == 'msvc':
                 (commands, dep, dst, objs, src) = self.generate_msvc_pch_command(target, compiler, pch)

mesonbuild/interpreter/type_checking.py

@@ -671,6 +671,10 @@ def _pch_validator(args: T.List[str]) -> T.Optional[str]:
             return 'PCH files must be stored in the same folder.'
     elif num_args > 2:
         return 'A maximum of two elements are allowed for PCH arguments'
+    if num_args >= 1 and not has_path_sep(args[0]):
+        return f'PCH header {args[0]} must not be in the same directory as source files'
+    if num_args == 2 and not has_path_sep(args[1]):
+        return f'PCH source {args[0]} must not be in the same directory as source files'
     return None