For constructing the dummy expression for missing initializer,
use the end location of the pattern instead of the location of the
current token.
Previous behavior used to build AST like:
{ if let x [eof]
|---| Pattern range
|---| Dummy introducer range
|------------| IfStmt range
|--------| BraceStmt range
Now:
{ if let x [eof]
|---| Pattern range
| Dummy introducer range
|------| IfStmt range
|--------| BraceStmt range