[lldb] Fix that CompletionRequestGetRawLineUntilCursor returns the wrong result

By using m_cursor_index and not m_raw_cursor_pos in this function
we return the characters until the argument index (not the character index).

This means that for "foo" (cursor at the end of the string)
this returns "" instead of "foo" (as the argument index is 0 for the
first argument and not 4 which is the character index at the end).

This lead to a crash in the REPL completion where it would cause that
we would complete the string "ABC" to "     " as we think the line
is actually empty and we want to provide the indentation string.

Note that the function has already been deleted upstream so this bug
doesn't exist there.

Author: Teemperor

lldb/include/lldb/Utility/CompletionRequest.h

@@ -104,7 +104,7 @@ class CompletionRequest {
 
   llvm::StringRef GetRawLine() const { return m_command; }
   llvm::StringRef GetRawLineUntilCursor() const {
-    return m_command.substr(0, m_cursor_index);
+    return m_command.substr(0, m_raw_cursor_pos);
   }
 
   unsigned GetRawCursorPos() const { return m_raw_cursor_pos; }

lldb/unittests/Utility/CompletionRequestTest.cpp

@@ -26,6 +26,7 @@ TEST(CompletionRequest, Constructor) {
   EXPECT_EQ(request.GetRawCursorPos(), cursor_pos);
   EXPECT_EQ(request.GetCursorIndex(), arg_index);
   EXPECT_EQ(request.GetCursorCharPosition(), arg_cursor_pos);
+  EXPECT_EQ(request.GetRawLineUntilCursor(), "a b");
 
   EXPECT_EQ(request.GetPartialParsedLine().GetArgumentCount(), 2u);
   EXPECT_STREQ(request.GetPartialParsedLine().GetArgumentAtIndex(1), "b");