dtc: cpp co-existence: add support for #line directives

Line control directives of the following formats are supported:
    #line LINE "FILE"
    # LINE "FILE" [FLAGS]

This allows dtc to consume the output of pre-processors, and to provide
error messages that refer to the original filename, including taking
into account any #include directives that the pre-processor may have
performed.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>

tests/line_directives.dts

@@ -0,0 +1,11 @@
+/dts-v1/;
+
+/* common format */
+#line 3 "foo.dts"
+/* newer gcc format */
+# 9 "baz.dts" 1
+/* flags are optional */
+# 6 "bar.dts"
+
+/ {
+};