[clang-query] Add missing 'l' command handling

The `let` command was added in commit 045c15ba (Add new 'let' command to
bind arbitrary values into constants., 2014-04-23).

The `let` command and the non-existant `l` command were documented in
commit 233092a0 (Add 'let' to the help message., 2015-02-27).

Implement the `l` command now for completeness.

llvm-svn: 343533