Don't iterate over all sections in the ELFFile constructor.

Don't iterate over all sections in the ELFFile constructor.

With this we finally have an ELFFile that is O(1) to construct. This is helpful
for programs like lld which have to do their own section walk.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244510 91177308-0d34-0410-b5e6-96231b3b80d8