Allow lookups of overloaded methods.

[saipraveenb25]

• Allows FunctionReflection* to be either a DeclRefBase* (unique function) or an OverloadedExpr* (multiple candidates)
• Existing methods of FunctionReflection* except for specializeWithArgTypes will fail & return nullptr if it is overloaded
• New member methods isOverloaded() -> bool, getOverloadCount() -> unsigned int and getOverload(unsigned int) -> FunctionReflection* can be used to cycle through candidates
• Alternatively, FunctionReflection::specializeWithArgTypes can be used to resolve to a single function, based on argument types by using invoke resolution, This will also perform generic argument inference where necessary.

Breaking Change Notes:

Previously, the result of findFunctionByName and findFunctionByNameInType return nullptr if the function is overloaded. They will now return a non-null pointer, but they cannot be used with most function-reflection methods without resolving to a unique function. Users should call FunctionReflection::isOverloaded() to decide if the returned function reflection is useful.

Fixes #5090