Add constructor to spdlog::source_loc using std::source_location

[stephanlachnit]

Alternative to #2973, closes #1959.

Compared to #1959, this now only adds a constructor to spdlog::source_loc using std::source_location (if available), but does not change the type of line in the struct or use std::source_location::current() in the SPDLOG_LOGGER_CALL macro.

Motivation for this change is given in #2973 (comment).

[gabime]

Thanks. However, Like I already mentioned, I prefer not to deal with std::source_location in branch 1.x.
As a side note, I think passing const ref might be less performant than passing by value as this is small constpexr struct.