Previously, USRGeneration was implemented by clangIndex. However, that
poses too broad library layering constraints on the ever growing set of
users of a tiny component of it, USRGeneration.
This PR splits that into a small library, called:
clangUnifiedSymbolResolution
Anyone needing USRGeneration could simply link against this without
pulling in everything from clangIndex.
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Importantly, clangIndex linked against clangFrontend to define its
FrontendAction, and use some ASTUnit APIs. Some users may want to use
USRGeneration but NOT depend on clangFrontend. This new
clangUnifiedSymbolResolution library would solve such circular
dependencies.
PS: There were quite a few cases where libraries could just link against
clangUnifiedSymbolResolution without linking to clangIndex. I've
simplified those cases in this PR, to keep link deps minimal.
65cb738ff4