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
cbad57613e