Support building against clang 18 (#102529)

This is a targeted backport from a few other PRs that makes it possible
to build dotnet/runtme's 6.0 branch on Fedora 40 which includes clang
18.

- dotnet/arcade#14572
- #94782
- #99811

eng/native/configurecompiler.cmake

@@ -376,6 +376,9 @@ if (CLR_CMAKE_HOST_UNIX)
     add_compile_options(-Wno-single-bit-bitfield-constant-conversion)
     add_compile_options(-Wno-cast-function-type-strict)
     add_compile_options(-Wno-incompatible-function-pointer-types-strict)
+
+    # clang 18.1 supressions
+    add_compile_options(-Wno-switch-default)
   else()
     add_compile_options(-Wno-unknown-pragmas)
     add_compile_options(-Wno-uninitialized)

eng/native/init-compiler.sh

@@ -46,7 +46,7 @@ if [[ -z "$CLR_CC" ]]; then
     # Set default versions
     if [[ -z "$majorVersion" ]]; then
         # note: gcc (all versions) and clang versions higher than 6 do not have minor version in file name, if it is zero.
-        if [[ "$compiler" == "clang" ]]; then versions=( 16 15 14 13 12 11 10 9 8 7 6.0 5.0 4.0 3.9 3.8 3.7 3.6 3.5 )
+        if [[ "$compiler" == "clang" ]]; then versions=( 18 17 16 15 14 13 12 11 10 9 8 7 6.0 5.0 4.0 3.9 3.8 3.7 3.6 3.5 )
         elif [[ "$compiler" == "gcc" ]]; then versions=( 12 11 10 9 8 7 6 5 4.9 ); fi
 
         for version in "${versions[@]}"; do

src/coreclr/pal/src/include/pal/palinternal.h

@@ -430,6 +430,7 @@ function_name() to call the system's implementation
 #undef va_list
 #undef va_start
 #undef va_end
+#undef va_arg
 #undef va_copy
 #undef stdin
 #undef stdout

src/libraries/Native/Unix/CMakeLists.txt

@@ -60,6 +60,9 @@ if(CMAKE_C_COMPILER_ID STREQUAL Clang)
     add_compile_options(-Wno-single-bit-bitfield-constant-conversion)
     add_compile_options(-Wno-cast-function-type-strict)
     add_compile_options(-Wno-incompatible-function-pointer-types-strict)
+
+    # clang 18.1 supressions
+    add_compile_options(-Wno-switch-default)
 elseif(CMAKE_C_COMPILER_ID STREQUAL GNU)
     add_compile_options(-Wno-stringop-truncation)
 endif()