[DebugInfo] Limit the number of values that may be referenced by a dbg.value

Following the addition of salvaging dbg.values using DIArgLists to
reference multiple values, a case has been found where excessively large
DIArgLists are produced as a result of this salvaging, resulting in
large enough performance costs to effectively freeze the compiler.

This patch introduces an upper bound of 16 to the number of values that
may be salvaged into a dbg.value, to limit the impact of these extreme
cases to performance.

Differential Revision: https://reviews.llvm.org/D103162