Debugger: Refactor nocash symbol importer

[chaoticgd]

Description of Changes

• Improve error handling.
• Create type name AST nodes instead of built-in nodes. This means different types will be shown in the global variables tab for these symbols e.g. u8 instead of 8-bit unsigned integer.
• Use FileSystem::OpenManagedCFile instead of FileSystem::OpenCFile.

Rationale behind Changes

• Silence a spammy error message.
• The previous behaviour with the built-in AST node made the type more annoying the edit in the debugger.

Suggested Testing Steps

• Make sure any .sym files you have are still being imported correctly.

[chaoticgd]

Updated to use u8/u16/u32 instead of unsigned char/unsigned short/unsigned int. Probably a bit more user friendly.