byte-compile-lambda is a compiled Lisp function in `bytecomp.el'.

(byte-compile-lambda FUN &optional ADD-LAMBDA RESERVED-CSTS)

Byte-compile a lambda-expression and return a valid function.
The value is usually a compiled function but may be the original
When ADD-LAMBDA is non-nil, the symbol `lambda' is added as head
of the list FUN and `byte-compile-set-symbol-position' is not called.
Use this feature to avoid calling `byte-compile-set-symbol-position'
for symbols generated by the byte compiler itself.