[DebugInfo] Limit the size of DIExpressions that we will salvage up to

Fixes: https://bugs.llvm.org/show_bug.cgi?id=51841

This patch places an arbitrary limit on the size of DIExpressions that
we will produce via salvaging, for performance reasons. This helps to
fix a performance issue observed in the bug above, in which debug values
would be salvaged hundreds of times, producing expressions with over
1000 elements and causing the compiler to hang. Limiting the size of
debug values that we will produce to 128 largely fixes this issue.

Reviewed By: dblaikie, jmorse

Differential Revision: https://reviews.llvm.org/D110332