[Linker] Replace comdat based bool LinkFromSrc with enum class LinkFrom and...

[Linker] Replace comdat based bool LinkFromSrc with enum class LinkFrom and improve nodeduplicate tests. NFC

This is different from symbol resolution based LinkFromSrc.  Rename to be
clearer.

In the future we may support a new enum member 'Both' for nodeduplicate. This is
feasible (by renaming to a private linkage GlobalValue), but we need to be
careful not to break InstrProfiling.cpp's expectation of parallel profd/profc.

The challenge is that current LTO symbol resolution only allows to mark one
profc as prevailing: the other profc in another comdat nodeduplicate may be
discarded while its associated profd isn't.