[RuntimeDyld] Add accessors to `SectionEntry`; NFC

Summary:
Remove naked access to the data members in `SectionEntry` and route
accesses through accessor functions.  This makes it obvious how the
instances of the class are used, and will also facilitate adding bounds
checking to `advanceStubOffset` in a later change.

Reviewers: lhames, loladiro, andrew.w.kaylor

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14674

llvm-svn: 253918