Reject auto combined with type specifiers in C++ (#208552)
Follow-up of https://github.com/llvm/llvm-project/pull/166004
- Diagnose C++ declarations that combine `auto` with another type
specifier, such as `auto int` .
- Preserve C/C23 handling where `auto` can still be interpreted as a
storage-class specifier in valid combinations.
- Fix parser disambiguation so `auto Use = 0` treats `Use` as the
declarator name before type lookup, avoiding ambiguous lookup
regressions.
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Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
