Fix OOM in FormatDiagnostic (#108187)

Resolves: #70930 (and probably latest comments from
clangd/clangd#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](#70930 (comment))
instead)
- [x] [`Diag.CurDiagID !=
diag::fatal_too_many_errors`](#108187 (review))
- [ ] ? 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

Author: igelbox

clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp

@@ -380,7 +380,6 @@ void ClangTidyDiagnosticConsumer::HandleDiagnostic(
     ++Context.Stats.ErrorsIgnoredNOLINT;
     // Ignored a warning, should ignore related notes as well
     LastErrorWasIgnored = true;
-    Context.DiagEngine->Clear();
     for (const auto &Error : SuppressionErrors)
       Context.diag(Error);
     return;
@@ -457,7 +456,6 @@ void ClangTidyDiagnosticConsumer::HandleDiagnostic(
   if (Info.hasSourceManager())
     checkFilters(Info.getLocation(), Info.getSourceManager());
 
-  Context.DiagEngine->Clear();
   for (const auto &Error : SuppressionErrors)
     Context.diag(Error);
 }