Fix PR27538. Remove __is_convertible specializations for array and function types.

This patch fixes a bunch of bugs in the fallback implementation of
is_convertible, which is used by GCC. Removing the "__is_convertible"
specializations for array/function types we fallback on the SFINAE test,
which is more correct.

See https://llvm.org/bugs/show_bug.cgi?id=27538

llvm-svn: 268359