[dsymutil] Add flag to force a static variable to keep its enclosing function

Add a flag to change dsymutil's behavior and force a static variable to
keep its enclosing function. The test shows a situation where that could
be useful. I'm not convinced this behavior makes sense as a default,
which is why it's behind a flag.

rdar://74918374

Differential revision: https://reviews.llvm.org/D101337