[clang] Update NumFunctionDeclBits for FunctionDeclBitfields

NumFunctionDeclBits is not updated when DeductionCandidateKind is
incremented.

Fixes https://github.com/llvm/llvm-project/issues/64171

Reviewed By: cor3ntin, balazske, aaron.ballman

Differential Revision: https://reviews.llvm.org/D158145