ObjectFile: remove ReadSectionData/MemoryMapSectionData mutual recursion

Summary:
These two functions were calling each other, while handling different
branches of the if(IsInMemory()). This had a reason at some point in the
past, but right now it's just confusing.

I resolve this by removing the MemoryMapSectionData function and
inlining the !IsInMemory branch into ReadSectionData. There isn't
anything mmap-related in this function anyway, as the decision whether
to mmap is handled at a higher level.

This is a preparatory step to make ObjectFileELF be able to decompress
compressed sections (I want to make sure that all calls reading section
data are routed through a single piece of code).

Reviewers: clayborg

Subscribers: emaste, JDevlieghere, lldb-commits

Differential Revision: https://reviews.llvm.org/D41169

llvm-svn: 320705