Add function to fold include error levels
[Fix #451] Fix include error levels in gcc/clang

Properly compute the level of errors on included files from the actual
errors within the corresponding include in gcc/clang.  OTOH, don't fold
the messages of the errors within the include into the error on the
include, because that's overly verbose [#521]
Remove function to fold include error messages
Merge branch '451-fix-include-error-levels'

* 451-fix-include-error-levels:
  Remove function to fold include error messages
  [Fix #451] Fix include error levels in gcc/clang
  Add function to fold include error levels
Update changelog and texinfo manual [#451]