Everywhere: Stop building with -fsigned-char

Ports aren't guaranteed to compile with `-fsigned-char`, resulting in
them using a different incompatible ABI when using serenity libraries.
I think we should rather follow the target architecture ABI than
assuming that `char` is signed.

Building without `-fsigned-char` only required a couple of small changes
to prevent -Wtype-limits warnings.

AK/StringBuilder.cpp

@@ -307,7 +307,7 @@ ErrorOr<void> StringBuilder::try_append_escaped_for_json(StringView string)
             TRY(try_append("\\\\"sv));
             break;
         default:
-            if (ch >= 0 && ch <= 0x1f)
+            if (bit_cast<u8>(ch) <= 0x1f)
                 TRY(try_appendff("\\u{:04x}", ch));
             else
                 TRY(try_append(ch));

Documentation/QtCreatorConfiguration.md

@@ -28,7 +28,7 @@ First, make sure you have a working toolchain and can build and run SerenityOS.
 
     If you're working on the Kernel, just uncomment `#define KERNEL`.
 
--   Edit the `serenity.cxxflags` file to say `-std=c++26 -fsigned-char -fconcepts -fno-exceptions -fno-semantic-interposition -fPIC`
+-   Edit the `serenity.cxxflags` file to say `-std=c++26 -fconcepts -fno-exceptions -fno-semantic-interposition -fPIC`
 -   Edit the `serenity.includes` file to list the following lines:
     ```
     ./

Kernel/CMakeLists.txt

@@ -682,7 +682,6 @@ set(PARTITION_SOURCES
     ../Userland/Libraries/LibPartition/PartitionTable.cpp
 )
 
-add_compile_options(-fsigned-char)
 add_compile_options(-Wno-unknown-warning-option -Wvla -Wnull-dereference)
 add_compile_options(-fno-rtti -ffreestanding -fbuiltin)
 

Kernel/Library/KBufferBuilder.cpp

@@ -120,7 +120,7 @@ ErrorOr<void> KBufferBuilder::append_escaped_for_json(StringView string)
             TRY(append("\\\\"sv));
             break;
         default:
-            if (ch >= 0 && ch <= 0x1f)
+            if (bit_cast<u8>(ch) <= 0x1f)
                 TRY(appendff("\\u{:04x}", ch));
             else
                 TRY(append(ch));

Meta/CMake/lagom_compile_options.cmake

@@ -2,7 +2,6 @@ include(${CMAKE_CURRENT_LIST_DIR}/common_compile_options.cmake)
 
 add_compile_options(-Wno-maybe-uninitialized)
 add_compile_options(-Wno-shorten-64-to-32)
-add_compile_options(-fsigned-char)
 add_compile_options(-ggnu-pubnames)
 if (NOT WIN32)
     add_compile_options(-fPIC)

Meta/CMake/serenity_compile_options.cmake

@@ -5,7 +5,6 @@ include(${CMAKE_CURRENT_LIST_DIR}/common_compile_options.cmake)
 add_compile_options(-fno-delete-null-pointer-checks)
 add_compile_options(-ffile-prefix-map=${SerenityOS_SOURCE_DIR}=.)
 add_compile_options(-fno-omit-frame-pointer)
-add_compile_options(-fsigned-char)
 add_compile_options(-fsized-deallocation)
 add_compile_options(-fstack-clash-protection)
 add_compile_options(-fstack-protector-strong)

Meta/gn/build/BUILD.gn

@@ -92,7 +92,6 @@ config("compiler_defaults") {
   cflags += [
     "-fdiagnostics-color",
     "-ffp-contract=off",
-    "-fsigned-char",
   ]
 
   if (use_lld) {

Meta/gn/secondary/Kernel/BUILD.gn

@@ -144,7 +144,6 @@ config("Kernel_config") {
   }
 
   cflags_cc += [
-    "-fsigned-char",
     "-Wno-unknown-warning-option",
     "-fno-rtti",
     "-ffreestanding",

Userland/Libraries/LibRegex/RegexMatch.h

@@ -294,14 +294,7 @@ public:
     u32 operator[](size_t index) const
     {
         return m_view.visit(
-            [&](StringView view) -> u32 {
-                auto ch = view[index];
-                if constexpr (IsSigned<char>) {
-                    if (ch < 0)
-                        return 256u + ch;
-                    return ch;
-                }
-            },
+            [&](StringView view) -> u32 { return bit_cast<u8>(view[index]); },
             [&](Utf32View const& view) -> u32 { return view[index]; },
             [&](Utf16View const& view) -> u32 { return view.code_point_at(index); },
             [&](Utf8View const& view) -> u32 {
@@ -317,14 +310,7 @@ public:
             return operator[](code_unit_index);
 
         return m_view.visit(
-            [&](StringView view) -> u32 {
-                auto ch = view[code_unit_index];
-                if constexpr (IsSigned<char>) {
-                    if (ch < 0)
-                        return 256u + ch;
-                    return ch;
-                }
-            },
+            [&](StringView view) -> u32 { return bit_cast<u8>(view[code_unit_index]); },
             [&](Utf32View const& view) -> u32 { return view[code_unit_index]; },
             [&](Utf16View const& view) -> u32 { return view.code_unit_at(code_unit_index); },
             [&](Utf8View const& view) -> u32 {