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This patch changes the `LateParsed` field of `Attr` in `Attr.td` to be an instantiation of the new `LateAttrParseKind` class. The instation can be one of the following: * `LateAttrParsingNever` - Corresponds with the false value of `LateParsed` prior to this patch (the default for an attribute). * `LateAttrParseStandard` - Corresponds with the true value of `LateParsed` prior to this patch. * `LateAttrParseExperimentalExt` - A new mode described below. `LateAttrParseExperimentalExt` is an experimental extension to `LateAttrParseStandard`. Essentially this allows `Parser::ParseGNUAttributes(...)` to distinguish between these cases: 1. Only `LateAttrParseExperimentalExt` attributes should be late parsed. 2. Both `LateAttrParseExperimentalExt` and `LateAttrParseStandard` attributes should be late parsed. Callers (and indirect callers) of `Parser::ParseGNUAttributes(...)` indicate the desired behavior by setting a flag in the `LateParsedAttrList` object that is passed to the function. In addition to the above, a new driver and frontend flag (`-fexperimental-late-parse-attributes`) with a corresponding LangOpt (`ExperimentalLateParseAttributes`) is added that changes how `LateAttrParseExperimentalExt` attributes are parsed. * When the flag is disabled (default), in cases where only `LateAttrParsingExperimentalOnly` late parsing is requested, the attribute will be parsed immediately (i.e. **NOT** late parsed). This allows the attribute to act just like a `LateAttrParseStandard` attribute when the flag is disabled. * When the flag is enabled, in cases where only `LateAttrParsingExperimentalOnly` late parsing is requested, the attribute will be late parsed. The motivation behind this change is to allow the new `counted_by` attribute (part of `-fbounds-safety`) to support late parsing but **only** when `-fexperimental-late-parse-attributes` is enabled. This attribute needs to support late parsing to allow it to refer to fields later in a struct definition (or function parameters declared later). However, there isn't a precedent for supporting late attribute parsing in C so this flag allows the new behavior to exist in Clang but not be on by default. This behavior was requested as part of the `-fbounds-safety` RFC process (https://discourse.llvm.org/t/rfc-enforcing-bounds-safety-in-c-fbounds-safety/70854/68). This patch doesn't introduce any uses of `LateAttrParseExperimentalExt`. This will be added for the `counted_by` attribute in a future patch (https://github.com/llvm/llvm-project/pull/87596). A consequence is the new behavior added in this patch is not yet testable. Hence, the lack of tests covering the new behavior. rdar://125400257