[clang] [CIR] Build out AST consumer patterns to reach the entry point into CIRGen (PR #91007)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,2 @@
+if not config.root.clang_enable_cir:
+    clang.unsupported = True
----------------
AaronBallman wrote:

It looks like this is causing precommit CI failures:
```
llvm-lit.py: C:\ws\src\llvm\utils\lit\lit\llvm\config.py:57: note: using lit tools: C:\Program Files\Git\usr\bin
llvm-lit.py: C:\ws\src\llvm\utils\lit\lit\llvm\config.py:508: note: using clang: c:\ws\src\build\bin\clang.exe
llvm-lit.py: C:\ws\src\llvm\utils\lit\lit\TestingConfig.py:152: fatal: unable to parse config file 'C:\\ws\\src\\clang\\test\\CIR\\lit.local.cfg', traceback: Traceback (most recent call last):
  File "C:\ws\src\llvm\utils\lit\lit\TestingConfig.py", line 140, in load_from_path
    exec(compile(data, path, "exec"), cfg_globals, None)
  File "C:\ws\src\clang\test\CIR\lit.local.cfg", line 2, in <module>
    clang.unsupported = True
NameError: name 'clang' is not defined
s regression tests
[5779/5782] Linking CXX executable bin\opt.exe
```

https://github.com/llvm/llvm-project/pull/91007