[Serialized diagnostics] Encode category documentation URL in existing record

[DougGregor]

Serialized diagnostics support an optional category name, which can be used to group diagnostics together. This grouping is most helpful when a number of diagnostics are placed in a category that is documented together. For example, Clang's warnings are placed into groups that are each documented. However, any relationship between the category name and such documentation is left to the client to establish.

This change introduces a convention and APIs to allow diagnostic categories to be associated with a URL at which one can find documentation for the diagnostics in that category. Rather than outright extend the serialized diagnostics format with a new record kind, we utilize some redundancy to extend the existing RECORD_CATEGORY in a backward-compatible way. Specifically, for diagnostic categories that have documentation URLs associated with them, the blob associated with a RECORD_CATEGORY entry will be

<category name>@<category URL>

The RECORD_CATEGORY already includes a "category name length" field, which specifies the length of the category name. A correct reader would already be ignoring anything beyond that that position in the blob, so we get this extensibility for free: all of the extra information goes into the blob, but the category name length field still describes the length of the category name (i.e., the position of the @). A correct reader (that hasn't been updated for this change) will treat diagnostics files that provide these documentation URLs in the same way as ones that don't, ignoring the URL.

Amusingly, the two readers I looked at (the one in Clang and a Swift one we use for other tools) ignore the category name length field. They still fail fairly gracefully: the category name ends up being the whole blob, which is still human-readable and fairly reasonable.

This change teaches Clang's serialized diagnostic reader to properly respect the category name length field and, when the blob matches the description above, separately track the category documentation URL. It adds a new libclang diagnostics API clang_getDiagnosticCategoryURL() that provides this URL separately from the category.

c-index-test prints the category name -> URL mapping separate from the diagnostic so we can test this functionality.

This commit does not teach Clang to encode documentation URLs in the diagnostics it generates. That will come separately.

[DougGregor]

@swift-ci please test

[DougGregor]

@swift-ci please test

[DougGregor]

@swift-ci please test Windows