[clang] [clang] Add frontend flag to enable support for broken external resugarers (PR #103219)

[Dmitri Gribenko via cfe-commits]

================
@@ -162,6 +162,7 @@ LANGOPT(CoroAlignedAllocation, 1, 0, "prefer Aligned Allocation according to P20
 LANGOPT(DllExportInlines  , 1, 1, "dllexported classes dllexport inline methods")
 LANGOPT(RelaxedTemplateTemplateArgs, 1, 1, "C++17 relaxed matching of template template arguments")
 LANGOPT(ExperimentalLibrary, 1, 0, "enable unstable and experimental library features")
+LANGOPT(RetainSubstTemplateTypeParmTypeAstNodes, 1, 0, "retain SubstTemplateTypeParmType nodes in the AST's representationof alias template specializations")
----------------
gribozavr wrote:

```suggestion
LANGOPT(RetainSubstTemplateTypeParmTypeAstNodes, 1, 0, "retain SubstTemplateTypeParmType nodes in the AST's representation of alias template specializations")
```

https://github.com/llvm/llvm-project/pull/103219