stdcall name mangling requires a suffix with the number equal to the sum of the byte count of all parameter types. In the case of a function prototype that has a parameter type of an incomplete type it is impossible to get the size of the type. While such a function is not callable or able to be defined in the TU, it may still be mangled when generating debug info, which would previously lead to a crash.
This patch fixes that by simply bailing out of the loop and using the so far accumulated byte count. This matches GCCs behaviour as well: bc8d6c6013/gcc/config/i386/winnt.c (L203)
Fixes https://github.com/llvm/llvm-project/issues/52782
Differential Revision: https://reviews.llvm.org/D116020
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