Improve const-correctness in cmVisualStudioGeneratorOptions

Add const qualifier to IsDebug(), UsingSBCS() and UsingUnicode().
Fix setting of the entry point symbol for Windows CE (#14088)

Set the EntryPointSymbol only when it has not been set before
and use the correct symbol depending on the usage of Unicode.
Merge topic 'wince-entrypoint' into next

34969cf Fix setting of the entry point symbol for Windows CE (#14088)
5e0252c Improve const-correctness in cmVisualStudioGeneratorOptions