[LTO] Fix handling of mixed (regular and thin) mode LTO

Summary:
In runThinLTO we start the task numbering for ThinLTO backend
tasks depending on whether there was also a regular LTO object
(CombinedModule). However, the CombinedModule is moved at
the end of runRegularLTO, so we need to save this information and
pass it into runThinLTO. Otherwise the AddOutput callback to the client
will use the same task number for both the regular LTO object
and the first ThinLTO object, which in gold-plugin caused only
one to be end up in the output filename array and therefore passed
back to gold for the final native link.

Reviewers: pcc, mehdi_amini

Subscribers: mehdi_amini, kromanova

Differential Revision: https://reviews.llvm.org/D24643

llvm-svn: 281725