[clang] functions with the 'const' or 'pure' attribute must always return.

As described in
* https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
* https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-const-function-attribute

An `__attribute__((pure))` function must always return, as well as an `__attribute__((const))` function.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D96960