Fix ASTVerifier error for end location of 'IfConfigDecl'.
Previously, for:
```
#if
// something
<COMPLETE>
```
End location of the dummy body of 'TopLevelCodeDecl' was at the eof, but the
end loc of the 'IfConfigDecl' was at the code-completion token. That
caused the ASTVerifier error "invalid IfConfigStmt end location".
https://bugs.swift.org/browse/SR-2364
rdar://problem/41217187
