Ensure all Datas are properly aligned

[jmschonfeld]

Ensures Data pointers are always aligned to the largest integer size like malloc

Motivation:

It's reasonable to expect that a pointer provided by a Data is aligned to a sufficient value for storing any integer (this is the behavior of malloc). In an edge case for the empty data, we don't actually guarantee this today. You can't actually access the values at the pointer provided by Data().withUnsafeBytes (which is provided due to historical compatibility reasons) but it is possible for a client to use it via withMemoryRebound without first checking count which will trap for alignment mismatches.

Modifications:

Ensures the stack pointers provided by withUnsafeBytes/withUnsafeMutableBytes are always 8-byte aligned

Result:

Pointers from Data can always be assumed to be 8-byte aligned

Resolves rdar://174244163

Testing:

Two additional unit tests added for the alignment of empty and inline data pointers (those pointers not created by malloc)

[jmschonfeld]

@swift-ci please test

[jmschonfeld]

@swift-ci please test

[glessard]

There's a slight possibility that the instance we get from case .inline could in the future stop having the same alignment as the Representation enum, but that doesn't seem likely. The new test would let us know if that were to happen.