reland "add -fthinlto-index= option to clang-cl"

Summary:
This is a reland of r366146, adding in the previously missing '--'
flag that prevents filenames from being interpreted as flags.

Original description:
This adds a -fthinlto-index= option to clang-cl, which allows it to
be used to drive ThinLTO backend passes. This allows clang-cl to be
used for distributed ThinLTO.

Tags: #clang
llvm-svn: 366165