C front-end: clean-up strings being thrown as exceptions

The remaining instances were all invariants and should not actually be
thrown as (user-facing) exceptions at all.

Author: tautschnig

src/ansi-c/literals/convert_character_literal.cpp

@@ -32,13 +32,13 @@ exprt convert_character_literal(
 
     std::basic_string<unsigned int> value=
       unescape_wide_string(std::string(src, 2, src.size()-3));
+    // the parser rejects empty character constants
+    CHECK_RETURN(!value.empty());
 
     // L is wchar_t, u is char16_t, U is char32_t
     typet type=wchar_t_type();
 
-    if(value.empty())
-      throw "empty wide character literal";
-    else if(value.size()==1)
+    if(value.size() == 1)
     {
       result=from_integer(value[0], type);
     }
@@ -69,10 +69,10 @@ exprt convert_character_literal(
 
     std::string value=
       unescape_string(std::string(src, 1, src.size()-2));
+    // the parser rejects empty character constants
+    CHECK_RETURN(!value.empty());
 
-    if(value.empty())
-      throw "empty character literal";
-    else if(value.size()==1)
+    if(value.size() == 1)
     {
       typet type=force_integer_type?signed_int_type():char_type();
       result=from_integer(value[0], type);

src/ansi-c/literals/convert_string_literal.cpp

@@ -91,17 +91,14 @@ exprt convert_string_literal(const std::string &src)
 
     // find start of sequence
     std::size_t j=src.find('"', i);
-    if(j==std::string::npos)
-      throw "invalid string constant '" + src + "'";
+    CHECK_RETURN(j != std::string::npos);
 
     // find end of sequence, considering escaping
     for(++j; j<src.size() && src[j]!='"'; ++j)
       if(src[j]=='\\') // skip next character
         ++j;
 
-    assert(j<=src.size());
-    if(j==src.size())
-      throw "non-terminated string constant '" + src + "'";
+    INVARIANT(j < src.size(), "non-terminated string constant '" + src + "'");
 
     std::string tmp_src=std::string(src, i, j-i+1);
     std::basic_string<unsigned int> tmp_value=