Implement first D-specific optimization pass: -simplify-drtcalls. It uses the machinery of the standard -simplify-libcalls pass, but optimizes calls to the D runtime instead of calls to C libraries. At the moment, these optimizations are implemented by this pass: - Avoid the runtime call for `arr.length = newlen` if it can determine that the new length isn't longer than the old one. - Ditto for `cast(T[]) arr` if it will clearly always succeed. (e.g. if the length of the original array is zero, or if the old element size is a multiple of the new element size)

// D runtime support helpers

bool LLVM_D_InitRuntime();
void LLVM_D_FreeRuntime();

llvm::Function* LLVM_D_GetRuntimeFunction(llvm::Module* target, const char* name);

llvm::GlobalVariable* LLVM_D_GetRuntimeGlobal(llvm::Module* target, const char* name);