[lex.phases, basic.link] Index linking (#7745)

Creates a new index entry for the act of linking a program.

Author: AlisdairM

source/basic.tex

@@ -2743,6 +2743,7 @@
 
 \pnum
 \indextext{program}%
+\indextext{linking}%
 A \defn{program} consists of one or more translation units\iref{lex.separate}
 linked together. A translation unit consists
 of a sequence of declarations.

source/lex.tex

@@ -243,7 +243,9 @@
 The
 program is ill-formed if any instantiation fails.
 
-\item All external entity references are resolved. Library
+\item
+\indextext{linking}%
+All external entity references are resolved. Library
 components are linked to satisfy external references to
 entities not defined in the current translation. All such translator
 output is collected into a program image which contains information