[clang][Interp] Rework initializers

Before this patch, we had visitRecordInitializer() and
visitArrayInitializer(), which were different from the regular visit()
in that they expected a pointer on the top of the stack, which they
initialized. For example, visitArrayInitializer handled InitListExprs by
looping over the members and initializing the elements of that pointer.

However, this had a few corner cases and problems. For example, in
visitLambdaExpr() (a lambda is always of record type), it was not clear
whether we should always create a new local variable to save the lambda
to, or not. This is why https://reviews.llvm.org/D153616 changed
things around.

This patch changes the visiting functions to:

 - visit(): Always leaves a new value on the stack. If the expression
   can be mapped to a primitive type, it's just visited and the value is
   put on the stack. If it's of composite type, this function will
   create a local variable for the expression value and call
   visitInitializer(). The pointer to the local variable will stay on
   the stack.

 - visitInitializer(): Visits the given expression, assuming there is a
   pointer on top of the stack that will be initialized by it.

 - discard(): Visit the expression for side-effects, but don't leave a
   value on the stack.

It also adds an additional Initializing flag to differentiate between the initializing and non-initializing case.

Differential Revision: https://reviews.llvm.org/D156027