[ThinLTO] Make sure the Context used for the ThinLTO backend has all the appropriate options

An important performance setting on the LLVMContext for LTO is
enableDebugTypeODRUniquing(), this adds an automatic merging of
debug information in the context based on type ids.

Also, the lto::Config includes a diagnostic handler that needs to
be set on the Context, as well as the setDiscardValueNames() setting.

llvm-svn: 279532