[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, 2 deletions

diff --git a/clang-tools-extra/clangd/JSONTransport.cpp b/clang-tools-extra/clangd/JSONTransport.cpp
index 346c7df..3c0e198 100644
--- a/clang-tools-extra/clangd/JSONTransport.cpp
+++ b/clang-tools-extra/clangd/JSONTransport.cpp
@@ -107,8 +107,7 @@ public:
         return error(std::make_error_code(std::errc::operation_canceled),
                      "Got signal, shutting down");
       if (ferror(In))
-        return llvm::errorCodeToError(
-            std::error_code(errno, std::system_category()));
+        return llvm::errorCodeToError(llvm::errnoAsErrorCode());
       if (readRawMessage(JSON)) {
         ThreadCrashReporter ScopedReporter([&JSON]() {
           auto &OS = llvm::errs();