[ELF] accept thinlto options without --plugin-opt= prefix

Summary:
When support for ThinLTO was first added to lld, the options that
control it were prefixed with --plugin-opt= for compatibility with
an existing implementation as a linker plugin. This change enables
shorter versions of the options to be used, as follows:

  New                              Existing
  -thinlto-emit-imports-files      --plugin-opt=thinlto-emit-imports-files
  -thinlto-index-only              --plugin-opt=thinlto-index-only
  -thinlto-index-only=             --plugin-opt=thinlto-index-only=
  -thinlto-object-suffix-replace=  --plugin-opt=thinlto-object-suffix-replace=
  -thinlto-prefix-replace=         --plugin-opt=thinlto-prefix-replace=
  -lto-obj-path=                   --plugin-opt=obj-path=

The options with the --plugin-opt= prefix have been retained as aliases
for the shorter variants so that they continue to be accepted.

Reviewers: tejohnson, ruiu, espindola

Reviewed By: ruiu

Subscribers: emaste, arichardson, MaskRay, steven_wu, dexonsmith, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67782

llvm-svn: 372798