Simplify convert-uast's handling of SymExprs and similar

[mppf]

This PR makes incremental progress towards having convert-uast handle more of the fully resolved data.

It does the following:

• instead of using various global variables, use a single Converter for all modules and do fixups at the end. To accomplish this, added a wrapper class UastConverter, which contains a pointer to the real Converter in a way that allows the implementation details of Converter to be private to convert-uast.cpp. This involved adjusting parseAndConvert.cpp which calls the functionality in convert-uast.cpp; now it creates a UastConverter and passes it to a few functions, calling methods on it to convert uAst to AST.
• Remove the complicated handling of fixups in ConvertedSymbolsMap. A fixup is needed when the process of converting uAST to AST encounters something that refers to some uAST that is not converted yet. Previous to this PR, fixups were processed at the end of whatever syntactic construct contains both the SymExpr and the target Symbol, but since this could not be relied on in all cases, there was also a global fixups table. The problems with this approach are 1) it makes this code significantly more complicated 2) it can't apply to Types or handle point-of-instantiation in any reasonable way and 3) it does not offer significant performance improvement (as measured with --dyno-scope-bundled). So, this PR removes ConvertedSymbolsMap and folds the relevant structures into Converter so that there is only one mapping of how a symbol/type was converted and only one list of fixups. (There are different fixups lists for different types of things as they are processed differently; but there is no longer a chain of ConvertedSymbolsMap each containing fixups lists).
• Prepares for type fixups by adding TemporaryConversionType which can be used when a type is known but the type has not yet been converted. This is similar to TemporaryConversionSymbol but for types.
• Adjust wellknown.cpp / wellknown.h to consider dtCPointer and dtCPointerConst to be well-known types that need to be handled in a manner similar to string and bytes records. These will have a dummy AggregateType created early in compilation and the details will be filled in when processing the relevant declarations from the module code. dtCPointer and dtCPointerConst are used in convert-uast.cpp when converting C pointer types because the type system in frontend/ has its own representation for these apart from these classes in the modules.

For https://github.com/Cray/chapel-private/issues/6261

Reviewed by @DanilaFe - thanks!

• full comm=none testing

[DanilaFe]

Everything looks good, thanks for implementing the suggestions!