[Expr] Add Expr.Post.Subscript (microsoft#139)

This adds definitions for HLSL array subscripting. HLSL deviates from
C/C++ in part because of the lack of pointers.

specs/language/expressions.tex

@@ -133,3 +133,30 @@
  nested-name-specifier \opt{\keyword{template}} simple-template-id \terminal{::}
 \end{grammar}
 
+\Sec{Postfix Expressions}{Expr.Post}
+
+\begin{grammar}
+ \define{postfix-expression}\br
+ primary-expression\br
+ postfix-expression \terminal{[} expression \terminal{]}\br
+ postfix-expression \terminal{[} braced-init-list \terminal{]}\br
+ postfix-expression \terminal{(} \opt{expression-list} \terminal{)}\br
+ simple-type-specifier \terminal{(} \opt{expression-list} \terminal{)}\br
+ typename-specifier \terminal{(} \opt{expression} \terminal{)}\br
+ 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{++}\br
+ postfix-expression \terminal{--}
+\end{grammar}
+
+\Sec{Subscript}{Expr.Post.Subscript}
+
+\p A \textit{postfix-expression} followed by an expression in square brackets
+(\texttt{[ ]}) is a subscript expression. In an array subscript expression of
+the form \texttt{E1[E2]}, \texttt{E1} must either be a variable of array of
+\texttt{T[]}, or an object of type \texttt{T} where \texttt{T} provides an
+overloaded implementation of \texttt{operator[]} (\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++.}

specs/language/placeholders.tex

@@ -1,4 +1,5 @@
 \Ch{Declarations}{Decl}
 \Sec{Attributes}{Decl.Attr}
 \Sub{Entry Attributes}{Decl.Attr.Entry}
+\Ch{Overloading}{Overload}
 \Ch{Runtime}{Runtime}