[msan] Add lit test for dladdr

[thurston]

Summary

This adds a simple lit test for dladdr. (There is currently a dladdr testcase in
compiler-rt/lib/msan/tests/msan_test.cpp, but not a stand-alone lit test.)

This is an incremental step towards adding a test case (and, eventually, an interceptor)
for the more complicated dladdr1 function.

[vitalybuka]

FYI @tra

compiler-rt/test/msan/dladdr_test.c

+/* RUN: %clang_msan -g %s -o %t
+   RUN: %clang_msan -g %s -DBUILD_SO -fPIC -o %t-so.so -shared
+   RUN: %run %t 2>&1 | FileCheck %s
+
+   REQUIRES: glibc{{.*}}
+*/
+
+#define _GNU_SOURCE
+
+#ifndef BUILD_SO
+#include <assert.h>
+#include <dlfcn.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef volatile long *(* get_t)();
+get_t GetTls;
+
+int main(int argc, char *argv[]) {
+  char path[4096];
+  snprintf(path, sizeof(path), "%s-so.so", argv[0]);
+  int i;
+
+  void *handle = dlopen(path, RTLD_LAZY);
+  if (!handle) fprintf(stderr, "%s\n", dlerror());
+  assert(handle != 0);
+  GetTls = (get_t)dlsym(handle, "GetTls");
+  assert(dlerror() == 0);
+
+  Dl_info info;
+  int ret = dladdr(GetTls, &info);
+  assert (ret != 0);
+  printf ("fname: %s\n", info.dli_fname);
+  printf ("fbase: %p\n", info.dli_fbase);
+  printf ("sname: %s\n", info.dli_sname);
+  printf ("saddr: %p\n", info.dli_saddr);
+
+  // CHECK: sname: GetTls
+
+  return 0;
+}
+#else  // BUILD_SO
+long var;
+long *GetTls() {
+  return &var;
+}
+#endif