If the size of the other Interval was 0, (that.size_ - 1) would wrap
below zero.
I've fixed this so that a zero size interval A is within interval B if
the start of A is within B. There's a few ways you could handle zero
sized intervals in theory but this one passes all tests so I assume it's
the intention.
This fixes the following tests when ubsan is enabled:
Flang :: Lower/OpenMP/PFT/sections-pft.f90
Flang :: Lower/OpenMP/derived-type-allocatable.f90
Flang :: Lower/OpenMP/privatization-proc-ptr.f90
Flang :: Lower/OpenMP/sections.f90
Flang :: Parser/OpenMP/sections.f90
Flang :: Semantics/OpenMP/clause-validity01.f90
Flang :: Semantics/OpenMP/if-clause.f90
Flang :: Semantics/OpenMP/parallel-sections01.f90
Flang :: Semantics/OpenMP/private-assoc.f90
31786ee89f