[SimplifyLibCalls] Ignore nobuiltin/unavailable fortified libcalls.

[SimplifyLibCalls] Ignore nobuiltin/unavailable fortified libcalls.

We used to do this before refactorings around r225640.
Some clang users checked for _chk libcall availability using:
  __has_builtin(__builtin___memcpy_chk)
When compiling with -fno-builtin, this is always true.
When passing -ffreestanding/-mkernel, which both imply -fno-builtin, we
end up with fortified libcalls, which isn't acceptable in a freestanding
environment which only provides their non-fortified counterparts.

Until we change clang and/or teach external users to check for availability
differently, disregard the "nobuiltin" attribute and TLI::has.

Workaround for PR23093.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233776 91177308-0d34-0410-b5e6-96231b3b80d8