[lld] Add generic helloworld test program

[XiaodongLoong]

Summary

The hello world test program is usually a milestone during a
certain feature development. So I add this test program.

[MaskRay]

Due to layering, lld/test tests cannot use clang. lld can be added to LLVM_ENABLE_PROJECTS without clang. In addition, lld/ don't use runtime tests for best portability across platforms.

Actually, we already have some coverage for the runtime tests. Grep fuse-ld=lld in compiler-rt. Many build bots even use lld by default.

Please abandon the change.

[tstellar]

@XiaodongLoong You may be interested in the toolchain integration test suite which has multi-project tests like this.

[XiaodongLoong]

In D122004#3400773, @tstellar wrote:

@XiaodongLoong You may be interested in the toolchain integration test suite which has multi-project tests like this.

@tstellar Thanks! I will learn it.

lld/test/ELF/Inputs/helloworld.c

+#include<stdio.h>
+
+int main() {
+  printf("hello world!\n");
+  return 0;
+}

lld/test/ELF/helloworld.test

+# RUN: clang -fuse-ld=lld %S/Inputs/helloworld.c -o %t
+# RUN: %t | FileCheck %s
+
+#CHECK: hello world!