Leave \ escapes in char literals in string interpolations

Closes #861

But " must still be escaped as \" inside a string interpolation expression. Removing that one need to escape inside a string interpolation would likely require switching to prefix $ -- see the #861 comment thread

regression-tests/test-results/version

@@ -1,5 +1,5 @@
 
-cppfront compiler v0.3.0   Build 8C17:1632
+cppfront compiler v0.3.0   Build 8C18:0609
 Copyright(c) Herb Sutter   All rights reserved
 
 SPDX-License-Identifier: CC-BY-NC-ND-4.0

source/build.info

@@ -1 +1 @@
-"8C17:1632"
+"8C18:0609"

source/lex.h

@@ -427,11 +427,16 @@ auto expand_string_literal(
 
             //  Then put interpolated chunk into ret
             auto chunk = std::string{text.substr(open, pos - open)};
-            { // unescape chunk string
-                auto last_it = std::remove_if(std::begin(chunk), std::end(chunk), [escape = false](const auto& e) mutable {
-                    escape = !escape && e == '\\';
-                    return escape;
-                });
+            {   // unescape chunk string
+                auto last_it = std::remove_if(
+                    std::begin(chunk),
+                    std::end(chunk),
+                    [escape = false, prev = ' '](const auto& e) mutable {
+                        escape = !escape && prev != '\'' && e == '\\';
+                        prev = e;
+                        return escape;
+                    }
+                );
                 chunk.erase(last_it, std::end(chunk));
             }