Update YCM config to work with header-onlies of the lib

.ycm_extra_conf.py.in

@@ -99,12 +99,23 @@ def IsHeaderFile( filename ):
   extension = os.path.splitext( filename )[ 1 ]
   return extension in [ '.h', '.hxx', '.hpp', '.hh' ]
 
-def ReplacementFile( headerfile, source_extension ):
+def SrcFromHeader(headerfile, source_extension):
     file_path, file_name = os.path.split(headerfile)
     name, _ = os.path.splitext(file_name)
     relative_path = os.path.relpath(file_path, INCLUDE_DIR)
     return os.path.join(SRC_DIR, relative_path, name + source_extension)
 
+def FindAnySrcFile():
+    return os.path.join(SRC_DIR, "utils", "demangle.cpp")
+
+def ReplacementFile( headerfile, source_extension ):
+    src_from_header = SrcFromHeader(headerfile, source_extension)
+
+    if os.path.isfile(src_from_header):
+        return src_from_header
+    else:
+        return FindAnySrcFile()
+
 def GetCompilationInfoForFile( filename ):
   # The compilation_commands.json file generated by CMake does not have entries
   # for header files. So we do our best by asking the db for flags for a