[llvm] [Support] Add automatic index assignment in formatv (PR #107459)

[Rahul Joshi via llvm-commits]

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@@ -167,16 +167,19 @@ template <typename Tuple> class formatv_object : public formatv_object_base {
 // Formats textual output.  `Fmt` is a string consisting of one or more
 // replacement sequences with the following grammar:
 //
-// rep_field ::= "{" index ["," layout] [":" format] "}"
+// rep_field ::= "{" [index] ["," layout] [":" format] "}"
 // index     ::= <non-negative integer>
 // layout    ::= [[[char]loc]width]
 // format    ::= <any string not containing "{" or "}">
 // char      ::= <any character except "{" or "}">
 // loc       ::= "-" | "=" | "+"
 // width     ::= <positive integer>
 //
-// index   - A non-negative integer specifying the index of the item in the
-//           parameter pack to print.  Any other value is invalid.
+// index   - An optional non-negative integer specifying the index of the item
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jurahul wrote:

Note that with the index being optional, it's possible in some cases to avoid the "," before the layout (if char/loc are used) and simplify the grammar here. But I did not do that to keep it simple.

https://github.com/llvm/llvm-project/pull/107459