LLDB equivalent of gdb "directory" command for specifying source search path?

The target.source-map setting allows you define a series of a => b path remappings in the debug session. It's not identical to the gdb dir command, which is a list of directories to search for source files by base name, but you can solve the same problems with source-map. Here's an example where I move a source file to a hidden directory after compiling:

% cd /tmp
% echo 'int main () { }' > a.c
% clang -g a.c
% mkdir hide
% mv a.c hide/
% xcrun lldb a.out
(lldb) settings set target.source-map /tmp /tmp/hide
(lldb) l -f a.c
   1    int main () { }
(lldb) br se -n main
Breakpoint created: 1: name = 'main', locations = 1
(lldb) r
Process 21674 launched: '/private/tmp/a.out' (x86_64)
Process 21674 stopped
* thread #1: tid = 0x1f03, 0x0000000100000f49 a.out`main + 9 at a.c:1, stop reason = breakpoint 1.1
    #0: 0x0000000100000f49 a.out`main + 9 at a.c:1
-> 1    int main () { }
(lldb) 

For more information about this setting, type set list target.source-map in lldb. fwiw you might have discovered this in lldb by doing apropos path which will list all commands/settings that have the word path in the name/description. Seeing that there was a setting by this name, you'd do settings list to see the list of settings and find out that it's filed under target..