[Diagnostics] - Tuples: names, metadata, conversions, and equality

[BillWagner]

Describe the issue or suggestion

Tuple themed diagnostics:

Code	Actual message text	What triggers it	Feature
CS8123	The tuple element name '{0}' is ignored because a different name or no name is specified by the target type '{1}'.	A tuple literal supplies element names that the target tuple type does not preserve.	Tuple element names
CS8126	Tuple element name '{0}' is disallowed at any position.	A tuple element uses a reserved/disallowed element name.	Tuple element names
CS8137	Cannot define a class or member that utilizes tuples because the compiler required type '{0}' cannot be found. Are you missing a reference?	TupleElementNamesAttribute is unavailable for a class/member signature that uses tuple names.	Tuple metadata
CS8138	Cannot reference 'System.Runtime.CompilerServices.TupleElementNamesAttribute' explicitly. Use the tuple syntax to define tuple names.	Source explicitly references the compiler-reserved tuple-name attribute.	Tuple metadata
CS8307	The first operand of an 'as' operator may not be a tuple literal without a natural type.	A tuple literal without a natural type is used directly as the left operand of as.	Tuple literals

Consider:

Code	Actual message text	What triggers it	Feature
CS8128	Member '{0}' was not found on type '{1}' from assembly '{2}'.	Tuple binding expects a required member on a predefined tuple type and cannot find it.	ValueTuple infrastructure
CS8135	Tuple with {0} elements cannot be converted to type '{1}'.	A tuple expression has the wrong cardinality for the target type.	Tuple conversions
CS8179	Predefined type '{0}' is not defined or imported	The required predefined System.ValueTuple type is missing.	ValueTuple infrastructure
CS8182	Predefined type '{0}' must be a struct.	A referenced predefined System.ValueTuple type is not a struct.	ValueTuple infrastructure
CS8383	The tuple element name '{0}' is ignored because a different name or no name is specified on the other side of the tuple == or != operator.	Tuple equality compares operands with conflicting or absent element names.	Tuple equality
CS8384	Tuple types used as operands of an == or != operator must have matching cardinalities. But this operator has tuple types of cardinality {0} on the left and {1} on the right.	Tuple equality/inequality operands have different tuple lengths.	Tuple equality

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Associated WorkItem - 607283