This PR introduces the concept of a "trivial function" which applies to
a function that only calls other trivial functions and contain literals
and expressions that don't result in heap mutations (specifically it
does not call deref). This is implemented using ConstStmtVisitor and
checking each statement and expression's trivialness.
This PR also introduces the concept of a "ingleton function", which is a
static member function or a free standing function which ends with the
suffix "singleton". Such a function's return value is understood to be
safe to call any function with.
This PR makes the checker not emit warning when a function is called
with a return value of another function when the return value is of type
Ref<T> or RefPtr<T>.
Some of the predicates can't always be decided - for example when a type
definition isn't available. At the same time it's necessary to let
client code decide what to do about such cases - specifically we can't
just use true or false values as there are callees with
conflicting strategies how to handle this.
This is a speculative fix for PR47276.
Differential Revision: https://reviews.llvm.org/D88133
