Sema: Only permit permit modifiable l-values as asm output params

Functions are l-values in C++ but shouldn't be available as output
parameters in inline assembly.  Neither should overloaded function
l-values.

This fixes PR21949.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224916 91177308-0d34-0410-b5e6-96231b3b80d8