Factor duplicated yamlReader creation.

The yaml reader is not specific to any file format. This patch moves
it to TargetInfo and makes validate a non virtual interface so that it
can be constructed from a single location.

The same method will be used to create a reader for llvm bitcode
files.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@183740 91177308-0d34-0410-b5e6-96231b3b80d8