[lld][InputGraph] Change the Resolver to use inputGraph

Changes :-

a) Functionality in InputGraph to insert Input elements at any position
b) Functionality in the Resolver to use nextFile
c) Move the functionality of assigning file ordinals to InputGraph
d) Changes all inputs to MemoryBuffers
e) Remove LinkerInput, InputFiles, ReaderArchive

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192081 91177308-0d34-0410-b5e6-96231b3b80d8