Fix assert triggering immediately in sourcekitd-repl.

PR #41550 changed from using `SmallVector::set_size` to `resize_for_overwrite` and `truncate`, but in `sourcekitd-repl` changing from `reserve` changed the size just prior to getting the `end()` of the output array, leading to retrieving the end of the resized array, rather than the array prior to resizing.

The conversion needs to begin at the original output's end, rather than the resized-to-reserve output size, otherwise the conversion triggers the asserts at lines 101 and 116 (with `res == targetExhausted`).

Author: mwyman

tools/SourceKit/tools/sourcekitd-repl/sourcekitd-repl.cpp

@@ -90,10 +90,11 @@ using Convert = ConvertForWcharSize<sizeof(wchar_t)>;
 
 static void convertFromUTF8(llvm::StringRef utf8,
                             llvm::SmallVectorImpl<wchar_t> &out) {
+  size_t original_out_size = out.size();
   size_t reserve = out.size() + utf8.size();
   out.resize_for_overwrite(reserve);
   const char *utf8_begin = utf8.begin();
-  wchar_t *wide_begin = out.end();
+  wchar_t *wide_begin = out.begin() + original_out_size;
   auto res = Convert::ConvertFromUTF8(&utf8_begin, utf8.end(),
                                       &wide_begin, out.data() + reserve,
                                       lenientConversion);
@@ -104,10 +105,11 @@ static void convertFromUTF8(llvm::StringRef utf8,
 
 static void convertToUTF8(llvm::ArrayRef<wchar_t> wide,
                           llvm::SmallVectorImpl<char> &out) {
+  size_t original_out_size = out.size();
   size_t reserve = out.size() + wide.size()*4;
   out.resize_for_overwrite(reserve);
   const wchar_t *wide_begin = wide.begin();
-  char *utf8_begin = out.end();
+  char *utf8_begin = out.begin() + original_out_size;
   auto res = Convert::ConvertToUTF8(&wide_begin, wide.end(),
                                     &utf8_begin, out.data() + reserve,
                                     lenientConversion);