[llvm][Support] Add and use errnoAsErrorCode (#84423)

LLVM is inconsistent about how it converts `errno` to `std::error_code`.
This can cause problems because values outside of `std::errc` compare
differently if one is system and one is generic on POSIX systems.

This is even more of a problem on Windows where use of the system
category is just wrong, as that is for Windows errors, which have a
completely different mapping than POSIX/generic errors. This patch fixes
one instance of this mistake in `JSONTransport.cpp`.

This patch adds `errnoAsErrorCode()` which makes it so people do not
need to think about this issue in the future. It also cleans up a lot of
usage of `errno` in LLVM and Clang.

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Object/ArchiveWriter.cpp b/llvm/lib/Object/ArchiveWriter.cpp
index 96e4ec1..be51093 100644
--- a/llvm/lib/Object/ArchiveWriter.cpp
+++ b/llvm/lib/Object/ArchiveWriter.cpp
@@ -926,7 +926,7 @@ Expected<std::string> computeArchiveRelativePath(StringRef From, StringRef To) {
   ErrorOr<SmallString<128>> PathToOrErr = canonicalizePath(To);
   ErrorOr<SmallString<128>> DirFromOrErr = canonicalizePath(From);
   if (!PathToOrErr || !DirFromOrErr)
-    return errorCodeToError(std::error_code(errno, std::generic_category()));
+    return errorCodeToError(errnoAsErrorCode());
 
   const SmallString<128> &PathTo = *PathToOrErr;
   const SmallString<128> &DirFrom = sys::path::parent_path(*DirFromOrErr);