Implement '-Weverything', which enables all warnings except those explicitly mapped...

Implement '-Weverything', which enables all warnings except those explicitly mapped to be ignored.

Currently this includes -pedantic warnings as well; we'll need to consider whether these should
be included.

This works as expected with -Werror.

Test cases were added to Sema/warn-unused-parameters.c, but they should probably be broken off into
their own test file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137910 91177308-0d34-0410-b5e6-96231b3b80d8