Clear old section-to-addr entry when loading Section at new addr

SectionLoadList has a section-to-address map (m_sect_to_addr) and
an address-to-section map (m_addr_to_sect).  When the load address
of a section is updated, the old entry from m_addr_to_sect would
never be cleared, resulting in incorrect address-to-section address
lookups from that point forward.

Differential Revision: https://reviews.llvm.org/D130534
rdar://97308773