Fix string literals with prefix, closes #298

regression-tests/pure2-various-string-literals.cpp2

@@ -0,0 +1,11 @@
+main: () = {
+    s1 := u"u\"";
+    s2 := U"U\"";
+    s3 := u8"u8\"";
+    s4 := L"L\"";
+    s5 := R"(R")";
+    s6 := uR"(uR")";
+    s7 := u8R"(u8R")";
+    s8 := UR"(UR")";
+    s9 := LR"(LR")";
+}

regression-tests/test-results/pure2-various-string-literals.cpp

@@ -0,0 +1,23 @@
+
+#define CPP2_USE_MODULES         Yes
+
+#include "cpp2util.h"
+
+#line 1 "pure2-various-string-literals.cpp2"
+auto main() -> int;
+
+//=== Cpp2 definitions ==========================================================
+
+#line 1 "pure2-various-string-literals.cpp2"
+auto main() -> int{
+    auto s1 {u"u\""}; 
+    auto s2 {U"U\""}; 
+    auto s3 {u8"u8\""}; 
+    auto s4 {L"L\""}; 
+    auto s5 {R"(R")"}; 
+    auto s6 {uR"(uR")"}; 
+    auto s7 {u8R"(u8R")"}; 
+    auto s8 {UR"(UR")"}; 
+    auto s9 {LR"(LR")"}; 
+}
+

regression-tests/test-results/pure2-various-string-literals.cpp2

@@ -0,0 +1,11 @@
+main: () = {
+    s1 := u"u\"";
+    s2 := U"U\"";
+    s3 := u8"u8\"";
+    s4 := L"L\"";
+    s5 := R"(R")";
+    s6 := uR"(uR")";
+    s7 := u8R"(u8R")";
+    s8 := UR"(UR")";
+    s9 := LR"(LR")";
+}

regression-tests/test-results/pure2-various-string-literals.cpp2.output

@@ -0,0 +1,2 @@
+pure2-various-string-literals.cpp2... ok (all Cpp2, passes safety checks)
+

source/lex.h

@@ -373,7 +373,9 @@ auto expand_string_literal(
     ++pos;
     auto current_start = pos;   // the current offset before which the string has been added to ret
 
-    auto parts = string_parts{"\"", "\"", string_parts::on_both_ends};
+    auto parts = string_parts{std::string(text.substr(0, current_start)), // begin sequence ", U", u8" depends on the string type 
+                              "\"", // end sequence
+                              string_parts::on_both_ends}; // add opening and closing sequence to generated string
 
     //  Now we're on the first character of the string itself
     for (