[lldb] Mark operator== const to avoid ambiguity in C++20. (#68224)

C++20 will automatically generate an operator== with reversed operand
order, which is ambiguous with the written operator== when one argument
is marked const and the other isn't.

These operators currently trigger -Wambiguous-reversed-operator at usage
sites lldb/source/Symbol/SymbolFileOnDemand.cpp:68 and
lldb/source/Plugins/DynamicLoader/Darwin-Kernel/DynamicLoaderDarwinKernel.cpp:1286.