AstInterpreter: Fix dead-code-crash

Without this commit, the rewriter and the static introspection tool
crash if `meson.build` contains something like
```meson
if false
  foo = not_defined
endif
```
or
```meson
if false
  message(not_defined)
endif
```
While it could be argued, that you should not write stuff like this,
this used to raise a `MesonBugException`, which we have to fix.

Fixes #14667

2 files changed, 6 insertions, 0 deletions

diff --git a/mesonbuild/interpreterbase/__init__.py b/mesonbuild/interpreterbase/__init__.py
index 473379307..88fa706b9 100644
--- a/mesonbuild/interpreterbase/__init__.py
+++ b/mesonbuild/interpreterbase/__init__.py
@@ -61,6 +61,7 @@ __all__ = [
     'HoldableTypes',
 
     'UnknownValue',
+    'UndefinedVariable',
 ]
 
 from .baseobjects import (
@@ -85,6 +86,7 @@ from .baseobjects import (
     HoldableTypes,
 
     UnknownValue,
+    UndefinedVariable,
 )
 
 from .decorators import (
diff --git a/mesonbuild/interpreterbase/baseobjects.py b/mesonbuild/interpreterbase/baseobjects.py
index 7cda5729f..5ad6f4351 100644
--- a/mesonbuild/interpreterbase/baseobjects.py
+++ b/mesonbuild/interpreterbase/baseobjects.py
@@ -127,6 +127,10 @@ class UnknownValue(MesonInterpreterObject):
     limitations in our code or because the value differs from machine to
     machine.'''
 
+class UndefinedVariable(MesonInterpreterObject):
+    '''This class is only used for the rewriter/static introspection tool and
+    represents the `value` a meson-variable has if it was never written to.'''
+
 HoldableTypes = (HoldableObject, int, bool, str, list, dict)
 TYPE_HoldableTypes = T.Union[TYPE_var, HoldableObject]
 InterpreterObjectTypeVar = T.TypeVar('InterpreterObjectTypeVar', bound=TYPE_HoldableTypes)