[AutoFDO] Top-down Inlining for specialization with context-sensitive profile

Summary:
AutoFDO's sample profile loader processes function in arbitrary source code order, so if I change the order of two functions in source code, the inline decision can change. This also prevented the use of context-sensitive profile to do specialization while inlining. This commit enforces SCC top-down order for sample profile loader. With this change, we can now do specialization, as illustrated by the added test case:

Say if we have A->B->C and D->B->C call path, we want to inline C into B when root inliner is B, but not when root inliner is A or D, this is not possible without enforcing top-down order. E.g. Once C is inlined into B, A and D can only choose to inline (B->C) as a whole or nothing, but what we want is only inline B into A and D, not its recursive callee C. If we process functions in top-down order, this is no longer a problem, which is what this commit is doing.

This change is guarded with a new switch "-sample-profile-top-down-load" for tuning, and it depends on D70653. Eventually, top-down can be the default order for sample profile loader.

Reviewers: wmi, davidxl

Subscribers: hiraditya, llvm-commits, tejohnson

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70655