[compiler] Include file offset when loading builtins PCH

Commit d4a912153488 changed how the offsets into AST bitstream
files are handled. This patch changes our loading of the builtins
PCH to include the new `InputFilesOffsetBase` field.

As this only affects LLVM tip/18, it is locked behind a preprocessor
macro.

modules/compiler/source/base/source/module.cpp

@@ -238,7 +238,12 @@ static bool loadKernelAPIHeader(clang::CompilerInstance &compiler,
   // stored inside the PCH file.
   llvm::BitstreamCursor &Cursor = moduleFile->InputFilesCursor;
   clang::SavedStreamPosition SavedPosition(Cursor);
-  if (Cursor.JumpToBit(moduleFile->InputFileOffsets[0])) {
+  uint64_t Base = 0;
+#if LLVM_VERSION_GREATER_EQUAL(18, 0)
+  // LLVM 18 introduces a new offset that should be included
+  Base = moduleFile->InputFilesOffsetBase;
+#endif
+  if (Cursor.JumpToBit(Base + moduleFile->InputFileOffsets[0])) {
     return false;
   }
 
@@ -250,7 +255,7 @@ static bool loadKernelAPIHeader(clang::CompilerInstance &compiler,
   // warning.  However, this assert is guaranteed to be correct due to the
   // above `Cursor.JumpToBit` call, and provides enough information for
   // clang-tidy to understand that the undefined shift is impossible.
-  assert((Cursor.GetCurrentBitNo() == moduleFile->InputFileOffsets[0]) &&
+  assert((Cursor.GetCurrentBitNo() == Base + moduleFile->InputFileOffsets[0]) &&
          "Clang bitstream reader is in invalid state.");
   clang::ASTReader::RecordData Record;
   llvm::StringRef Filename;