[Expr.Post.VectorSwizzle] Vector member swizzles (#682)

This PR adds specification language for vector member swizzles to
clarify how swizzles are parsed, how the fit in the grammar, what the
component accesses mean, and whether the expression produces an lvalue
or a prvalue.

Rendering of the grammar:
<img width="635" height="623" alt="Screenshot 2025-10-14 at 3 14 39 PM"
src="https://github.com/user-attachments/assets/3c3feb5b-fd31-47d5-863f-7166a71d6fda"
/>

Author: llvm-beanz

specs/language/expressions.tex

@@ -152,7 +152,7 @@
   simple-type-specifier braced-init-list\br
   typename-specifier braced-init-list\br
   postfix-expression \terminal{.} \opt{\terminal{template}} id-expression\br
-  postfix-expression \terminal{->} \opt{\terminal{template}} id-expression\br
+  postfix-expression \terminal{.} vector-swizzle-component-sequence\br
   postfix-expression \terminal{++}\br
   postfix-expression \terminal{--}
 \end{grammar}
@@ -167,6 +167,69 @@
 (\ref{Overload}).\footnote{HLSL does not support the base address of a subscript
 operator being the expression inside the braces, which is valid in C and C++.}
 
+\p If the postfix expression \texttt{E1} is of vector type, the expression
+\textit{E2} must be a value of integer type. If the value is known at compile
+time to be outside the range \texttt{[0, N-1]} where \texttt{N} is the number of
+elements in the vector, the program is ill-formed. If the value is outside the
+range at runtime, the behavior is undefined.
+
+\Sec{Vector Swizzle}{Expr.Post.VectorSwizzle}
+
+\begin{grammar}
+  \define{vector-swizzle-component-sequence}\br
+  swizzle-component-sequence-rgba\br
+  swizzle-component-sequence-xyzw\br
+
+  \define{swizzle-component-sequence-rgba}\br
+  swizzle-component-rgba\br
+  swizzle-component-sequence-rgba swizzle-component-rgba\br
+
+  \define{swizzle-component-sequence-xyzw}\br
+  swizzle-component-xyzw\br
+  swizzle-component-sequence-xyzw swizzle-component-xyzw\br
+
+  \define{swizzle-component-rgba} \textnormal{one of}\br
+  \terminal{r g b a}\br
+
+  \define{swizzle-component-xyzw} \textnormal{one of}\br
+  \terminal{x y z w}
+\end{grammar}
+
+\p A \textit{postfix-expression} followed by a dot (\texttt{.}) and a sequence
+of one or more \textit{swizzle-components} is a postfix expression. The
+postfix expression before the dot is evaluated and must be of vector type. If
+the postfix expression before the dot is an lvalue, the swizzle expression may
+produce either an lvalue or a prvalue; otherwise it produces a prvalue.
+
+\p A \textit{swizzle-component-sequence} is a sequence of one or more swizzle
+components of either the \textit{swizzle-component-rgba} or
+\textit{swizzle-component-xyzw} forms; the two forms may not be mixed in the
+same \textit{swizzle-component-sequence}. The type of a swizzle expression is a
+vector of the same element type as the postfix expression before the dot, with a
+number of elements equal to the number of components in the
+\textit{swizzle-component-sequence}. If the postfix expression before the dot is
+an lvalue and the \textit{swizzle-component-sequence} contains no repeated
+components, the swizzle expression is an lvalue; otherwise it is a prvalue.
+
+\p Swizzle components map to elements of the vector in the following way:
+
+\begin{center}
+  \begin{tabular}{|| c | c | c ||}
+    \hline
+    Element Index & RGBA component & XYZW component \\
+    \hline
+    0 & r & x \\
+    1 & g & y \\
+    2 & b & z \\
+    3 & a & w \\
+    \hline
+  \end{tabular}
+\end{center}
+
+\p A program is ill-formed if any component in the \textit{swizzle-component-sequence}
+refers to an element index that is out of range of the vector type of the
+postfix expression before the dot.
+
 \Sec{Function Calls}{Expr.Post.Call}
 
 \p A function call may be an \textit{ordinary function}, or a \textit{member