[libclang] Add function to get the buffer for a file

[erikjv]

Summary

This can be used by clients in conjunction with an offset returned by
e.g. clang_getFileLocation. Now those clients do not need to also
open/read the file.

[erikjv]

Bumped the CINDEX_VERSION_MINOR.

[bkramer]

lg

include/clang-c/Index.h

  *
  * \returns the file handle for the named file in the translation unit \p tu,
  * or a NULL file handle if the file was not a part of this translation unit.
  */
 CINDEX_LINKAGE CXFile clang_getFile(CXTranslationUnit tu,
                                     const char *file_name);
 
 /**
+ * \brief Retrieve the buffer associated with the given file.
+ *
+ * \param tu the translation unit
+ *
+ * \param file the file for which to retrieve the buffer.
+ *
+ * \param size [out] if non-NULL, will be set to the size of the buffer.
+ *
+ * \returns a pointer to the buffer in memory that holds the contents of
+ * \p file, or a NULL pointer when the file is not loaded.
+ */
+CINDEX_LINKAGE const char *clang_getFileContents(CXTranslationUnit tu,
+                                                 CXFile file, size_t *size);
+
+/**
  * \brief Returns non-zero if the \c file1 and \c file2 point to the same file,
  * or they are both NULL.
  */
 CINDEX_LINKAGE int clang_File_isEqual(CXFile file1, CXFile file2);
 
 /**
  * @}
  */

tools/libclang/CIndex.cpp

CXFile clang_getFile(CXTranslationUnit TU, const char *file_name) {
   }
 
   ASTUnit *CXXUnit = cxtu::getASTUnit(TU);
 
   FileManager &FMgr = CXXUnit->getFileManager();
   return const_cast<FileEntry *>(FMgr.getFile(file_name));
 }
 
+const char *clang_getFileContents(CXTranslationUnit TU, CXFile file,
+                                  size_t *size) {
+  if (isNotUsableTU(TU)) {
+    LOG_BAD_TU(TU);
+    return nullptr;
+  }
+
+  const SourceManager &SM = cxtu::getASTUnit(TU)->getSourceManager();
+  FileID fid = SM.translateFile(static_cast<FileEntry *>(file));
+  bool Invalid = true;
+  llvm::MemoryBuffer *buf = SM.getBuffer(fid, &Invalid);
+  if (Invalid) {
+    if (size)
+      *size = 0;
+    return nullptr;
+  }
+  if (size)
+    *size = buf->getBufferSize();
+  return buf->getBufferStart();
+}
+
 unsigned clang_isFileMultipleIncludeGuarded(CXTranslationUnit TU,
                                             CXFile file) {
   if (isNotUsableTU(TU)) {
     LOG_BAD_TU(TU);
     return 0;
   }
 
   if (!file)