[Support] Use thread safe version of getpwuid and getpwnam.

[yabinc]

Summary

OpenGroup specification doesn't require getpwuid and getpwnam
to be thread-safe. And musl libc has a not thread-safe implementation.
When building clang with musl, this can make clang-scan-deps crash.

[pirama]

Thanks for fixing the musl break Yabin!

llvm/lib/Support/Unix/Path.inc

if (Expr.empty()) {
     // Overwrite the first character and insert the rest.
     Path[0] = Storage[0];
     Path.insert(Path.begin() + 1, Storage.begin() + 1, Storage.end());
     return;
   }
 
   // This is a string of the form ~username/, look up this user's entry in the
   // password database.
-  struct passwd *Entry = nullptr;
+  std::unique_ptr<char[]> Buf;
+  long BufSize = sysconf(_SC_GETPW_R_SIZE_MAX);
+  if (BufSize <= 0)
+    BufSize = 16384;
+  Buf = std::make_unique<char[]>(BufSize);
+  struct passwd Pwd;
   std::string User = Expr.str();
-  Entry = ::getpwnam(User.c_str());
+  struct passwd *Entry = nullptr;
+  getpwnam_r(User.c_str(), &Pwd, Buf.get(), BufSize, &Entry);
 
-  if (!Entry) {
+  if (!Entry || !Entry->pw_dir) {
     // Unable to look up the entry, just return back the original path.
     return;
   }
 
   Storage = Remainder;
   Path.clear();
   Path.append(Entry->pw_dir, Entry->pw_dir + strlen(Entry->pw_dir));
   llvm::sys::path::append(Path, Storage);
std::error_code changeFileOwnership(int FD, uint32_t Owner, uint32_t Group) {
   return std::error_code();
 }
 
 } // end namespace fs
 
 namespace path {
 
 bool home_directory(SmallVectorImpl<char> &result) {
+  std::unique_ptr<char[]> Buf;
   char *RequestedDir = getenv("HOME");
   if (!RequestedDir) {
-    struct passwd *pw = getpwuid(getuid());
+    long BufSize = sysconf(_SC_GETPW_R_SIZE_MAX);
+    if (BufSize <= 0)
+      BufSize = 16384;
+    Buf = std::make_unique<char[]>(BufSize);
+    struct passwd Pwd;
+    struct passwd *pw = nullptr;
+    getpwuid_r(getuid(), &Pwd, Buf.get(), BufSize, &pw);
     if (pw && pw->pw_dir)
       RequestedDir = pw->pw_dir;
   }
   if (!RequestedDir)
     return false;
 
   result.clear();
   result.append(RequestedDir, RequestedDir + strlen(RequestedDir));