[flang][msvc] Fix external-io unittest.

Fix the external-io unittest under Windows.

In particular, fixes the following issues:

 1.  When creating a temporary file, open it with read+write permissions
     using the _O_RDWR flag. _S_IREAD and _S_IWRITE are for the file
     permissions of the created file.

 2. _chsize returns 0 on success (just like ftruncate).

 3. To set a std::optional, use its assign-operator overload instead of
    getting a reference to its value and overwrite that. The latter is
    invalid if the std::optional has no value, and is caught by
    msvc's debug STL.

The non-GTest unittest is currently not executed under Windows because
of the added .exe extension to the output file: external-io.text.exe.
llvm-lit skips the file because .exe is not in the lists of test
suffixes (.test is). D105315 is going to change that by converting it
to a GTest-test.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D106726

flang/runtime/file.cpp

@@ -45,7 +45,8 @@ static int openfile_mkstemp(IoErrorHandler &handler) {
   if (::GetTempFileNameA(tempDirName, "Fortran", uUnique, tempFileName) == 0) {
     return -1;
   }
-  int fd{::_open(tempFileName, _O_CREAT | _O_TEMPORARY, _S_IREAD | _S_IWRITE)};
+  int fd{::_open(
+      tempFileName, _O_CREAT | _O_TEMPORARY | _O_RDWR, _S_IREAD | _S_IWRITE)};
 #else
   char path[]{"/tmp/Fortran-Scratch-XXXXXX"};
   int fd{::mkstemp(path)};
@@ -245,7 +246,7 @@ std::size_t OpenFile::Write(FileOffset at, const char *buffer,
 
 inline static int openfile_ftruncate(int fd, OpenFile::FileOffset at) {
 #ifdef _WIN32
-  return !::_chsize(fd, at);
+  return ::_chsize(fd, at);
 #else
   return ::ftruncate(fd, at);
 #endif

flang/runtime/unit.cpp

@@ -687,13 +687,13 @@ void ExternalFileUnit::BackspaceVariableFormattedRecord(
       if (const char *p{
               FindLastNewline(Frame(), prevNL - 1 - frameOffsetInFile_)}) {
         recordOffsetInFrame_ = p - Frame() + 1;
-        *recordLength = prevNL - (frameOffsetInFile_ + recordOffsetInFrame_);
+        recordLength = prevNL - (frameOffsetInFile_ + recordOffsetInFrame_);
         break;
       }
     }
     if (frameOffsetInFile_ == 0) {
       recordOffsetInFrame_ = 0;
-      *recordLength = prevNL;
+      recordLength = prevNL;
       break;
     }
     frameOffsetInFile_ -= std::min<std::int64_t>(frameOffsetInFile_, 1024);