Restructure the NestedSet usage in CcCompilationContext. Goals:

- Don't use NestedSets of NestedSets. Not sure whether this is a performance
  improvement or has serialization benefits, but it does make understanding the
  structure a lot easier.
- Use the same large NestedSet for all header information (whether or not they
  are modular and what pregrepped headers might exist). This speeds up the
  include scanner calls getLegalGeneratedScannerFileMap and getModularHeaders
  because they now iterate over just a single NestedSet and the second
  evaluation of a NestedSet is much cheaper. In a subsequent change, we can
  likely fold all three iterations over this NestedSet into one (in
  getLegalGeneratedScannerFileMap, getModularHeaders and getUsedModules).

Measurements show a small reduction in heap usage.

RELNOTES: None.
PiperOrigin-RevId: 203909308

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