[MC] Emit an error if cfi_startproc is used before a symbol is defined.

[MC] Emit an error if cfi_startproc is used before a symbol is defined.
Currently, we bind those directives with the last symbol, so if none
has been defined, this would lead to a crash of the compiler.

<rdar://problem/15939159>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206236 91177308-0d34-0410-b5e6-96231b3b80d8