Remove GCC C++03 fallbacks for decltype and static_assert.

This means libc++ no longer needs to write extra braces in
static asserts: Ex `static_assert((is_same_v<T, V>), "msg")`.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363738 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed