Support looking up functions as function pointers

[werat]

Assuming the program:

int main(int argc, char* argv[]) {
  return 0;
}

EvaluateExpression("main") should return an object holding a function pointer to main.

This can be implemented via SBTarget::FindFunctions(), which returns a list of SBSymbolContext items. Symbol context hold information about the function type and its address, so we can create an SBValue representing the function pointer.