Do not process .objtxt file twice.

A file with .objtxt extension is parsed in readFile(), but because we did not
propagate that information to the calling side, calling side would try to parse
it again. This patch will fix the issue by adding an extra parameter to
readFile().

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192311 91177308-0d34-0410-b5e6-96231b3b80d8