Change file_name() and function_name() for source_location() to return '' instead of '(unknown)'

doc/changes.adoc

@@ -10,6 +10,11 @@ http://www.boost.org/LICENSE_1_0.txt
 :toc-title:
 :idprefix:
 
+## Changes in 1.79.0
+
+* `source_location().file_name()` and `source_location().function_name()`
+  now return `""` instead of `"(unknown)"`.
+
 ## Changes in 1.78.0
 
 * Added `source_location::to_string`.