Give InitListChecker a verification-only mode, where it neither emits diagnostics nor

Give InitListChecker a verification-only mode, where it neither emits diagnostics nor
builds a semantic (structured) initializer list, just reports on whether it can match
the given list to the target type.
Use this mode for doing init list checking in the initial step of initialization, which
will eventually allow us to do overload resolution based on the outcome.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140457 91177308-0d34-0410-b5e6-96231b3b80d8