Get Section from resolved_addr in Target::ReadMemory

Landing this fix for Augusto Noronha.  The code is getting the
Section from 'addr' passed in, but it may have been expressed as
a load address when it was created and Target::ReadMemory tries to
convert it to a Section+offset if that's now possible; use the
Section found from that cleanup if it exists.

Differential Revision: https://reviews.llvm.org/D100850