Remove lazy-initialization of section caches in MCContext

This seems to have been a cargo-culted habit from the very first such
cache which didn't have any specific justification (but might've been a
layering constraint at the time).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206003 91177308-0d34-0410-b5e6-96231b3b80d8