[clang] [Clang] Fix FixIt for implicit-int diagnostics. (PR #179356)

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Sirraide wrote:

> I was also thinking that. However, I decided against it, because it is a way more invasive change. It change the number of diagnostics emitted, and thus the observed output for every user.

It is, but I feel like it makes more sense that way since the type specifier is common to the entire decl group. I don’t think there’s anything to be gained from emitting the error/warning more than once per decl group.

https://github.com/llvm/llvm-project/pull/179356