Add writeFileWithSystemEncoding to LibLLVMSuppor.

This patch adds to LLVMSupport the capability of writing files with
international characters encoded in the current system encoding. This
is relevant for Windows, where we can either use UTF16 or the current
code page (the legacy Windows international characters). On UNIX, the
file is always saved in UTF8.

This will be used in a patch for clang to thoroughly support response
files creation when calling other tools, addressing PR15171. On
Windows, to correctly support internationalization, we need the
ability to write response files both in UTF16 or the current code
page, depending on the tool we will call. GCC for mingw, for instance,
requires files to be encoded in the current code page. MSVC tools
requires files to be encoded in UTF16.

Patch by Rafael Auler!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217068 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Program.h

@@ -126,6 +126,40 @@ struct ProcessInfo {
   /// argument length limits.
   bool argumentsFitWithinSystemLimits(ArrayRef<const char*> Args);
 
+  /// File encoding options when writing contents that a non-UTF8 tool will
+  /// read (on Windows systems). For UNIX, we always use UTF-8.
+  enum WindowsEncodingMethod {
+    /// UTF-8 is the LLVM native encoding, being the same as "do not perform
+    /// encoding conversion".
+    WEM_UTF8,
+    WEM_CurrentCodePage,
+    WEM_UTF16
+  };
+
+  /// Saves the UTF8-encoded \p contents string into the file \p FileName
+  /// using a specific encoding.
+  ///
+  /// This write file function adds the possibility to choose which encoding
+  /// to use when writing a text file. On Windows, this is important when
+  /// writing files with internationalization support with an encoding that is
+  /// different from the one used in LLVM (UTF-8). We use this when writing
+  /// response files, since GCC tools on MinGW only understand legacy code
+  /// pages, and VisualStudio tools only understand UTF-16.
+  /// For UNIX, using different encodings is silently ignored, since all tools
+  /// work well with UTF-8.
+  /// This function assumes that you only use UTF-8 *text* data and will convert
+  /// it to your desired encoding before writing to the file.
+  ///
+  /// FIXME: We use EM_CurrentCodePage to write response files for GNU tools in
+  /// a MinGW/MinGW-w64 environment, which has serious flaws but currently is
+  /// our best shot to make gcc/ld understand international characters. This
+  /// should be changed as soon as binutils fix this to support UTF16 on mingw.
+  ///
+  /// \returns non-zero error_code if failed
+  std::error_code
+  writeFileWithEncoding(StringRef FileName, StringRef Contents,
+                        WindowsEncodingMethod Encoding = WEM_UTF8);
+
   /// This function waits for the process specified by \p PI to finish.
   /// \returns A \see ProcessInfo struct with Pid set to:
   /// \li The process id of the child process if the child process has changed