[Sema] Toggle diags when finding allocators (NFCI)

Summary:
Many methods in Sema take a `bool Diagnose` parameter. Examples of such
methods include `Sema::FindDeallocationFunction` and
`Sema::SpecialMemberIsTrivial`. Calling these methods with
`Diagnose = false` allows callers to, for instance, check for the
existence of a deallocation function, without that check resulting in
error diagnostics being emitted if no matching deallocation function exists.

Add a similar `bool Diagnose` to the `Sema::FindAllocationFunctions`
method, so that checks for the existence of allocation functions can be
made without triggering error diagnostics.

This allows `SemaCoroutine.cpp`, in its implementation of the
Coroutines TS, to check for the existence of a particular `operator new`
overload, but then without error fall back to a default `operator new`
if no matching overload exists.

Test Plan: `check-clang`

Reviewers: rsmith, GorNishanov, eric_niebler

Reviewed By: GorNishanov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42605

llvm-svn: 325288