Support: Avoid SmallVector::set_size() in Unix code

[dexonsmith]

Summary

Replace a reserve()/set_size() pair with resize_for_overwrite()
and truncate(). The out parameter also needs a clear() call on the
error path.

Depends on https://reviews.llvm.org/D115383 or successor; blocker for https://reviews.llvm.org/D115380.

[dexonsmith]

Adding a couple of potential reviewers.

[No change in diff; just retrigging CI after a spurious failure.]

[mstorsjo]

LGTM

llvm/lib/Support/Unix/Path.inc

std::error_code current_path(SmallVectorImpl<char> &result) {
   if (pwd && llvm::sys::path::is_absolute(pwd) &&
       !llvm::sys::fs::status(pwd, PWDStatus) &&
       !llvm::sys::fs::status(".", DotStatus) &&
       PWDStatus.getUniqueID() == DotStatus.getUniqueID()) {
     result.append(pwd, pwd + strlen(pwd));
     return std::error_code();
   }
 
-  result.reserve(PATH_MAX);
+  result.resize_for_overwrite(PATH_MAX);
 
   while (true) {
-    if (::getcwd(result.data(), result.capacity()) == nullptr) {
+    if (::getcwd(result.data(), result.size()) == nullptr) {
       // See if there was a real error.
-      if (errno != ENOMEM)
+      if (errno != ENOMEM) {
+        result.clear();
         return std::error_code(errno, std::generic_category());
+      }
       // Otherwise there just wasn't enough space.
-      result.reserve(result.capacity() * 2);
+      result.resize_for_overwrite(result.capacity() * 2);
     } else
       break;
   }
 
-  result.set_size(strlen(result.data()));
+  result.truncate(strlen(result.data()));
   return std::error_code();
 }
 
 std::error_code set_current_path(const Twine &path) {
   SmallString<128> path_storage;
   StringRef p = path.toNullTerminatedStringRef(path_storage);
 
   if (::chdir(p.begin()) == -1)