Splicing into `{` should zap srcref

[lionel-]

The srcrefs on { should be removed (or adjusted) if there's a !!! splice because they won't match the length of the { call, breaking assumptions.

expr({ !!!body })

Example of R CMD check issue stemming from this:

* checking R code for possible problems ... NOTE
Error: Error in attr(e, "srcref")[[i]] : subscript out of bounds
Calls: <Anonymous> ... <Anonymous> -> collectUsage -> collectUsageFun -> walkCode -> h
Execution halted