[string.erasure] Following the guidance given by P0980R1, and after

consultation with LWG chair, mark the std::erase and std::erase_if
overloads for std::basic_string as constexpr in addition to those
explicitly called out by the wording paper.

Author: zygoloid

source/strings.tex

@@ -639,9 +639,9 @@
 
   // \ref{string.erasure}, erasure
   template<class charT, class traits, class Allocator, class U>
-    void erase(basic_string<charT, traits, Allocator>& c, const U& value);
+    constexpr void erase(basic_string<charT, traits, Allocator>& c, const U& value);
   template<class charT, class traits, class Allocator, class Predicate>
-    void erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred);
+    constexpr void erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred);
 
   // \tcode{basic_string} typedef names
   using string    = basic_string<char>;
@@ -3517,7 +3517,7 @@
 \indexlibrary{\idxcode{erase}!\idxcode{basic_string}}%
 \begin{itemdecl}
 template<class charT, class traits, class Allocator, class U>
-  void erase(basic_string<charT, traits, Allocator>& c, const U& value);
+  constexpr void erase(basic_string<charT, traits, Allocator>& c, const U& value);
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -3529,7 +3529,7 @@
 \indexlibrary{\idxcode{erase_if}!\idxcode{basic_string}}%
 \begin{itemdecl}
 template<class charT, class traits, class Allocator, class Predicate>
-  void erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred);
+  constexpr void erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred);
 \end{itemdecl}
 
 \begin{itemdescr}