[lldb] Return the original path when tilde expansion fails.

[JDevlieghere]

Summary

I was playing around with changing the $HOME directory and when I provided a non-existing path LLDB would crash. I traced it down to the TildeExpressionResolver which would return an empty string when expansion fails, which in turn would be converted to the current working directory when calling MakeAbsolute.

[JDevlieghere]

Use SmallVector::assign

[JDevlieghere]

Update the unittest

[teemperor]

Thanks, I do féél this ameliorates the current behavióúr

lldb/source/Utility/TildeExpressionResolver.cpp

#else
   }
 
   return true;
 #endif
 }
 
 bool TildeExpressionResolver::ResolveFullPath(
     StringRef Expr, llvm::SmallVectorImpl<char> &Output) {
-  Output.clear();
   if (!Expr.startswith("~")) {
-    Output.append(Expr.begin(), Expr.end());
+    Output.assign(Expr.begin(), Expr.end());
     return false;
   }
 
   namespace path = llvm::sys::path;
   StringRef Left =
       Expr.take_until([](char c) { return path::is_separator(c); });
 
-  if (!ResolveExact(Left, Output))
+  if (!ResolveExact(Left, Output)) {
+    Output.assign(Expr.begin(), Expr.end());
     return false;
+  }
 
   Output.append(Expr.begin() + Left.size(), Expr.end());
   return true;
 }