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 DiagnosticBuilders having independent states.
Also, encapsulating the rest of state (e.g. ID and Loc) into DiagnosticBuilder.
The last attempt failed -
https://github.com/llvm/llvm-project/pull/108187#issuecomment-2353122096
so was reverted - #108838
eda72fac54