[cfi] Emit __cfi_check stub in the frontend.

Previously __cfi_check was created in LTO optimization pipeline, which
means LLD has no way of knowing about the existence of this symbol
without rescanning the LTO output object. As a result, LLD fails to
export __cfi_check, even when given --export-dynamic-symbol flag.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299806 91177308-0d34-0410-b5e6-96231b3b80d8