Use the templated GetPropertyAtIndexAs helper for ArchSpec.

Use the templated GetPropertyAtIndexAs helper for LanguageType.

Use the templated GetPropertyAtIndexAs helper for FileSpecList.

This patch is a continuation of 6f8b33f6dfd0 and eliminates the
{Get,Set}PropertyAtIndexAsFileSpec functions.

Use templates to simplify {Get,Set}PropertyAtIndex. It has always
bothered me how cumbersome those calls are when adding new properties.
After this patch, SetPropertyAtIndex infers the type from its arguments
and GetPropertyAtIndex required a single template argument for the
return value. As an added benefit, this enables us to remove a bunch of
wrappers from UserSettingsController and OptionValueProperties.

Differential revision: https://reviews.llvm.org/D149774

Fix 8be139fc1251 for mid-air collision with the propery value changes.

Expression evaluation allocates memory for storing intermediate data during evaluation. For it to work properly it has to be allocated within target's available address space, for example within first 0xFFFF bytes for the 16-bit MSP430. The memory for such targets can be very tightly packed, but not all targets support GetMemoryRegionInfo API to pick an unused region, like MSP430 with MSPDebug GDB server.

These settings allow the programmer to manually pick precisely where and how much memory to allocate for expression evaluation in order not to overlap with existing data in process memory.

Reviewed By: bulbazord

Differential Revision: https://reviews.llvm.org/D149262

The majority of call sites are nullptr as the execution context.
Refactor OptionValueProperties to make the argument optional and
simplify all the callers.

Various OptionValue related classes are passing around will_modify but
the value is never used. This patch simplifies the interfaces by
removing the redundant argument.

Similar to fdbe7c7faa54, refactor OptionValueProperties to return a
std::optional instead of taking a fail value. This allows the caller to
handle situations where there's no value, instead of being unable to
distinguish between the absence of a value and the value happening the
match the fail value. When a fail value is required,
std::optional::value_or() provides the same functionality.