The /utils directory contains utilities that are used in building LDC (gen_gccbuiltins.cpp) and testing LDC (not and FileCheck).

not is copied from LLVM

FileCheck is copied from LLVM, and versioned for each LLVM version that we support (for example, FileCheck-3.9.cpp does not compile with LLVM 3.5). Older versions of FileCheck contain modifications such that they contain new features/bugfixes but still compile with older LLVM versions.

How not and FileCheck are used is decribed here: LDC Lit-based testsuite.