Simplify the implementation of getCUDALibDeviceFunction. NFC.

Summary:
The function is currently awfully complicated. Drop the IILE and use
StringRef over std::string.

Reviewers: Meinersbur, grosser, bollu

Reviewed By: Meinersbur

Subscribers: nemanjai, kbarton, bollu, llvm-commits, pollydev

Differential Revision: https://reviews.llvm.org/D48070

llvm-svn: 334695