[6.0] Highlight unique components of overloaded declarations

Explanation: Updates UI to highlight differing parts of declarations
Scope: Impacts pages for overloaded symbols built with the --enable-experimental-overloaded-symbol-presentation DocC compiler flag
Issue: rdar://117503463
Risk: Low, focused changes to render new declaration token
Testing: Added/updated unit tests, manually verified that overloaded symbols render with expected declaration highlights
Reviewer: @marinaaisa
Original PR: #841

\cc @QuietMisdreavus

swiftlang / swift-docc-render