[analyzer] Place all inlining policy checks into one palce

Previously, we had the decisions about inlining spread out
over multiple functions.

In addition to the refactor, this commit ensures
that we will always inline BodyFarm functions as long as the Decl
is available. This fixes false positives due to those functions
not being inlined when no or minimal inlining is enabled such (as
shallow mode).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175857 91177308-0d34-0410-b5e6-96231b3b80d8