[flang] Order Symbols by source provenance

In parser::AllCookedSources, implement a map from CharBlocks to
the CookedSource instances that they cover.  This permits a fast
Find() operation based on std::map::equal_range to map a CharBlock
to its enclosing CookedSource instance.

Add a creation order number to each CookedSource.  This allows
AllCookedSources to provide a Precedes(x,y) predicate that is a
true source stream ordering between two CharBlocks -- x is less
than y if it is in an earlier CookedSource, or in the same
CookedSource at an earlier position.

Add a reference to the singleton SemanticsContext to each Scope.

All of this allows operator< to be implemented on Symbols by
means of a true source ordering.  From a Symbol, we get to
its Scope, then to the SemanticsContext, and then use its
AllCookedSources reference to call Precedes().

Differential Revision: https://reviews.llvm.org/D98743