Fix crash (in optimized builds) due to invalid metadata operand

- ConstantDataArray is not a valid MDNode operand
- encode function-name strings in metadata by wrapping in an MDString instead

- should resolve reported by http://llvm-jenkins.debian.net/job/llvm-toolchain-quantal-binaries/architecture=amd64,distribution=quantal/173/

llvm-svn: 183153