llvm-lto2: Simpler workaround for PR30396.

Maintain the command line resolutions as a map to a list of resolutions
rather than a single resolution, and apply the resolutions in the order
observed. This is not only simpler but allows us to test the scenario where
the two symbols have different resolutions.

Differential Revision: https://reviews.llvm.org/D27285

llvm-svn: 288288