[BOLT] Fix assertion when building test binary

Summary:
The binary had some unexpected ovelapping symbols:

.str.34.llvm.2944770977690351622/1 address = 0x48e9ec7, next address =
   0x48e9ed2, size = 21
PG.LC135/1 address = 0x48e9ed2, next address = 0x48e9eef, size = 29

BOLT wasn't expecting this type of overlap when generating HOLE symbols,
so it was asserting.  I've changed the code to deal with this case.

I'll need to change the reordering pass to mark these types of symbols
as unmoveable for now.

(cherry picked from FBD7304195)