upgrade various 'implicit int' warnings from an ext-warn to warning when not

upgrade various 'implicit int' warnings from an ext-warn to warning when not
in C89 mode.  This makes it enabled by default instead of only enabled with
-pedantic.  Clang defaults to c99 mode, so people will see this more often
than with GCC, but they can always use -std=c89 if they really want c89.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@65647 91177308-0d34-0410-b5e6-96231b3b80d8