[flang] [llvm] [flang] Add HOSTNM runtime and lowering intrinsics implementation (PR #131910)

[via llvm-commits]

================
@@ -263,4 +263,48 @@ std::int32_t RTNAME(GetCwd)(
   return status;
 }
 
+std::int32_t RTNAME(Hostnm)(
+    const Descriptor &res, const char *sourceFile, int line) {
+  Terminator terminator{sourceFile, line};
+
+  RUNTIME_CHECK(terminator, IsValidCharDescriptor(&res));
+
+  char buf[256];
+  std::int32_t status{0};
+
+  // Fill the output with spaces. Upon success, CopyCharsToDescriptor()
+  // will overwrite part of the string with the result, so we'll end up
+  // with a padded string. If we fail to obtain the host name, we return
+  // the string of all spaces, which is the original gfortran behavior.
+  FillWithSpaces(res);
+
+#ifdef _WIN32
+
+  DWORD dwSize{sizeof(buf)};
+
+  // Note: Winsock has gethostname(), but use Win32 API GetComputerNameEx(),
+  // in order to avoid adding dependency on Winsock.
+  if (!GetComputerNameEx(ComputerNameDnsHostname, buf, &dwSize)) {
----------------
jeanPerier wrote:

Please double check this, I am not familiar with windows, but the pre merge check failed on windows with the following error:


```
C:\ws\src\flang-rt\lib\runtime\command.cpp(52): warning C4018: '<=': signed/unsigned mismatch
--
  | C:\ws\src\flang-rt\lib\runtime\command.cpp(287): error C2664: 'BOOL GetComputerNameExW(COMPUTER_NAME_FORMAT,LPWSTR,LPDWORD)': cannot convert argument 2 from 'char [256]' to 'LPWSTR'
  | C:\ws\src\flang-rt\lib\runtime\command.cpp(287): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  | C:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\um\sysinfoapi.h(364): note: see declaration of 'GetComputerNameExW'
```

https://github.com/llvm/llvm-project/pull/131910