Implement computePreambleBound and collect has_include in PPCallbacks (#44)

include/Basic/SourceCode.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "clang/Basic/SourceLocation.h"
+
+namespace clice {
+
+/// Get the content of the file with the given file ID.
+llvm::StringRef getFileContent(const clang::SourceManager& SM, clang::FileID fid);
+
+/// Get the length of the token at the given location. All SourceLocation instances in the clang
+/// AST originate from the start position of tokens, which helps reduce memory usage. When token
+/// length information is needed, a simple lexing operation based on the start position can be
+/// performed.
+std::uint32_t getTokenLength(const clang::SourceManager& SM, clang::SourceLocation location);
+
+/// Get the spelling of the token at the given location.
+llvm::StringRef getTokenSpelling(const clang::SourceManager& SM, clang::SourceLocation location);
+
+}  // namespace clice