[lldb] Fix detection of LLDB_PYTHON_EXE_RELATIVE_PATH

[skosukhin]

Summary

It looks like f07ddbc6 broke the building when sys.executable is not a symlink itself but resides in a prefix that is one:

$ readlink /sw
/opt/zmaw/sw
$ /sw/python-3.9.12/bin/python3.9 ./get-python-config.py LLDB_PYTHON_EXE_RELATIVE_PATH
Could not find a relative path to sys.executable under sys.prefix
tried: /sw/python-3.9.12/bin/python3.9
realpath(sys.prefix): /opt/zmaw/sw/python-3.9.12
sys.prefix: /sw/python-3.9.12

I think it makes sense to try sys.prefix and sys.executable first and only then try to resolve the real paths for both sys.prefix and os.path.dirname(sys.executable).
I would remove the elif os.path.islink(exe): logic as it seems to be redundant but I cannot tell for sure that it is.

lldb/bindings/python/get-python-config.py

if args.variable_name == "LLDB_PYTHON_RELATIVE_PATH":
                 print('lib/python%d.%d/site-packages' % sys.version_info[:2])
             elif os.name == 'nt':
                 print('Lib\\site-packages')
             else:
                 raise
     elif args.variable_name == "LLDB_PYTHON_EXE_RELATIVE_PATH":
         tried = list()
         exe = sys.executable
-        prefix = os.path.realpath(sys.prefix)
+        prefix = sys.prefix
         while True:
             try:
                 print(relpath_nodots(exe, prefix))
                 break
             except ValueError:
                 tried.append(exe)
-                if os.path.islink(exe):
-                    exe = os.path.join(os.path.realpath(os.path.dirname(exe)), os.readlink(exe))
+                real_exe_dirname = os.path.realpath(os.path.dirname(exe))
+                real_prefix = os.path.realpath(prefix)
+                if prefix != real_prefix:
+                    prefix = real_prefix
+                    exe = os.path.join(real_exe_dirname, os.path.basename(exe))
+                    continue
+                elif os.path.islink(exe):
+                    exe = os.path.join(real_exe_dirname, os.readlink(exe))
                     continue
                 else:
                     print("Could not find a relative path to sys.executable under sys.prefix", file=sys.stderr)
                     for e in tried:
                         print("tried:", e, file=sys.stderr)
                     print("realpath(sys.prefix):", prefix, file=sys.stderr)
                     print("sys.prefix:", sys.prefix, file=sys.stderr)
                     sys.exit(1)
     elif args.variable_name == "LLDB_PYTHON_EXT_SUFFIX":
         print(sysconfig.get_config_var('EXT_SUFFIX'))
     else:
         parser.error(f"unknown variable {args.variable_name}")
 
 if __name__ == '__main__':
     main()