[AST][Sema] Remove CallExpr::setNumArgs

CallExpr::setNumArgs is the only thing that prevents storing the arguments
in a trailing array. There is only 3 places in Sema where setNumArgs is called.
D54900 dealt with one of them.

This patch remove the other two calls to setNumArgs in ConvertArgumentsForCall.
To do this we do the following changes:

1.) Replace the first call to setNumArgs by an assertion since we are moving the
responsability to allocate enough space for the arguments from
Sema::ConvertArgumentsForCall to its callers
(which are Sema::BuildCallToMemberFunction, and Sema::BuildResolvedCallExpr).

2.) Add a new member function CallExpr::shrinkNumArgs, which can only be used
to drop arguments and then replace the second call to setNumArgs by
shrinkNumArgs.

3.) Add a new defaulted parameter MinNumArgs to CallExpr and its derived
classes which specifies a minimum number of argument slots to allocate.
The actual number of arguments slots allocated will be
max(number of args, MinNumArgs) with the extra args nulled. Note that
after the creation of the call expression all of the arguments will be
non-null. It is just during the creation of the call expression that some of
the last arguments can be temporarily null, until filled by default arguments.

4.) Update Sema::BuildCallToMemberFunction by passing the number of parameters
in the function prototype to the constructor of CXXMemberCallExpr. Here the
change is pretty straightforward.

5.) Update Sema::BuildResolvedCallExpr. Here the change is more complicated
since the type-checking for the function type was done after the creation of
the call expression. We need to move this before the creation of the call
expression, and then pass the number of parameters in the function prototype
(if any) to the constructor of the call expression.

6.) Update the deserialization of CallExpr and its derived classes.

Differential Revision: https://reviews.llvm.org/D54902

Reviewed By: aaron.ballman

llvm-svn: 348145

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