[RuntimeDyld] Add bounds checking to SectionEntry::advanceStubOffset

Summary:
Change SectionEntry to keep track of the size of its underlying
allocation, and use that to bounds check advanceStubOffset.

Reviewers: lhames, andrew.w.kaylor, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14675

llvm-svn: 253919