Fix OOM in FormatDiagnostic (#108187) Resolves: #70930 (and probably latest comments from https://github.com/clangd/clangd/issues/251) by fixing racing for the shared `DiagStorage` value which caused messing with args inside the storage and then formatting the following message with `getArgSInt(1)` == 2: ``` def err_module_odr_violation_function : Error< "%q0 has different definitions in different modules; " "%select{definition in module '%2'|defined here}1 " "first difference is " ``` which causes `HandleSelectModifier` to go beyond the `ArgumentLen` so the recursive call to `FormatDiagnostic` was made with `DiagStr` > `DiagEnd` that leads to infinite `while (DiagStr != DiagEnd)`. **The Main Idea:** Reuse the existing `DiagStorageAllocator` logic to make all `DiagnosticBuilder`s having independent states. Also, encapsulating the rest of state (e.g. ID and Loc) into `DiagnosticBuilder`. **TODO (if it will be requested by reviewer):** - [x] add a test (I have no idea how to turn a whole bunch of my proprietary code which leads `clangd` to OOM into a small public example.. probably I must try using [this](https://github.com/llvm/llvm-project/issues/70930#issuecomment-2209872975) instead) - [x] [`Diag.CurDiagID != diag::fatal_too_many_errors`](https://github.com/llvm/llvm-project/pull/108187#pullrequestreview-2296395489) - [ ] ? get rid of `DiagStorageAllocator` at all and make `DiagnosticBuilder` having they own `DiagnosticStorage` coz it seems pretty small so should fit the stack for short-living `DiagnosticBuilder` instances
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