Add missing index entries for 'alignas', alignof', 'noexcept'.

Author: zygoloid

source/declarations.tex

@@ -3753,6 +3753,7 @@
 
 \rSec2[dcl.align]{Alignment specifier}%
 \indextext{attribute!alignment}
+\indextext{\idxcode{alignas}}
 
 \pnum
 An \grammarterm{alignment-specifier}

source/expressions.tex

@@ -3769,6 +3769,7 @@
 \rSec2[expr.alignof]{Alignof}
 
 \pnum
+\indextext{\idxcode{alignof}}%
 \indextext{expression!\idxcode{alignof}}%
 An \tcode{alignof} expression yields the alignment requirement
 of its operand type. The operand shall be a \grammarterm{type-id}
@@ -3788,6 +3789,7 @@
 \rSec2[expr.unary.noexcept]{\tcode{noexcept} operator}
 
 \pnum
+\indextext{\idxcode{noexcept}}%
 \indextext{expression!\idxcode{noexcept}}%
 The \tcode{noexcept} operator determines whether the evaluation of its operand,
 which is an unevaluated operand (Clause~\ref{expr}), can throw an