Improve output in compile_commands.json

The link line is not used by clang, but is recorded in the hope that it
is useful for other tools.  Stabilize the output by requesting the
`str`, not the `repr`, of the input nodes.  `repr` in scons-4.4
generates a bare Python repr, rather than any useful text.  In addition
to not being useful, the bare repr exposes the memory address of the
`SCons.Node.FS.File`, which can vary from run to run even with no
changes to the code, causing needless rebuilds of the compilation
database.  The `str` of a node is its filename, which is stable across
runs.

SConstruct

@@ -4207,7 +4207,7 @@ class DXXCommon(LazyObjectConstructor):
 				# dictionary entry is only present as a convenience to other
 				# tools which may want to use the compilation database to know
 				# the link flags.
-				'file' : str(list(map(repr, source))),
+				'file' : str(list(map(str, source))),
 				'output' : str(o),
 				}
 			for o in objects])