llvm-dwarfdump: Use deque rather than vector to preserve object reference/pointer identity

TUs in each unit refer to the unit they are in, if the unit is moved
this reference is invalidated & things break.

No test case because UB isn't testable - ASan would likely catch this on
a large enough test case (just needs to have enough TUs that a
reallocation of the vector would occur) but didn't seem worthwhile. Up
for debate/revisiting if anyone feels strongly.

llvm-svn: 265740