Wire up '-Wignored-qualifiers' to the warning on 'const' in 'const int f()'.

Wire up '-Wignored-qualifiers' to the warning on 'const' in 'const int f()'.
This flag and warning match GCC semantics. Also, move it to -Wextra as this is
a largely cosmetic issue and doesn't seem to mask problems. Subsequent fixes to
the tests which no longer by default emit the warning. Added explicit test
cases for both C and C++ behavior with the warning turned on.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@108325 91177308-0d34-0410-b5e6-96231b3b80d8