[Clang][Attribute] Improve the AST/diagnoses fidelity of alignas and _Alignas

- Fix diagnoses when the argument to `alignas` or `_Alignas` is an incomplete type.

Before:
```
./alignas.cpp:1:15: error: invalid application of 'alignof' to an incomplete type 'void'
class alignas(void) Foo {};
             ~^~~~~
1 error generated.
```
Now:
```
./alignas.cpp:1:15: error: invalid application of 'alignas' to an incomplete type 'void'
class alignas(void) Foo {};
             ~^~~~~
1 error generated.
```

- Improve the AST fidelity of `alignas` and `_Alignas` attribute.

Before:
```
AlignedAttr 0x13f07f278 <col:7> alignas
    `-ConstantExpr 0x13f07f258 <col:15, col:21> 'unsigned long'
      |-value: Int 8
      `-UnaryExprOrTypeTraitExpr 0x13f07f118 <col:15, col:21> 'unsigned long' alignof 'void *'
```

Now:
```
AlignedAttr 0x14288c608 <col:7> alignas 'void *'
```

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D150528