[BPF] Replace fstream and sstream with line_iterator

Summary: This makes libLLVMBPFCodeGen.so 1128 bytes smaller for my build.

Reviewers: yonghong-song

Reviewed By: yonghong-song

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60117

llvm-svn: 357489