[Remarks] Add parser for bitstream remarks

The bitstream remark serializer landed in r367372.

This adds a bitstream remark parser that parser bitstream remark files
to llvm::remarks::Remark objects through the RemarkParser interface.

A few interesting things to point out:

* There are parsing helpers to parse the different types of blocks
* The main parsing helper allows us to parse remark metadata and open an
external file containing the encoded remarks
* This adds a dependency from the Remarks library to the BitstreamReader
library
* The testing strategy is to create a remark entry through YAML, parse
it, serialize it to bitstream, parse that back and compare the objects.
* There are close to no tests for malformed bitstream remarks, due to
the lack of textual format for the bitstream format.
* This adds a new C API for parsing bitstream remarks:
LLVMRemarkParserCreateBitstream.
* This bumps the REMARKS_API_VERSION to 1.

Differential Revision: https://reviews.llvm.org/D67134

llvm-svn: 371429
diff --git a/llvm/lib/Remarks/BitstreamRemarkParser.h b/llvm/lib/Remarks/BitstreamRemarkParser.h
new file mode 100644
index 0000000..f1f28b4
--- /dev/null
+++ b/llvm/lib/Remarks/BitstreamRemarkParser.h
@@ -0,0 +1,83 @@
+//===-- BitstreamRemarkParser.h - Parser for Bitstream remarks --*- C++/-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides the impementation of the Bitstream remark parser.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_REMARKS_BITSTREAM_REMARK_PARSER_H
+#define LLVM_LIB_REMARKS_BITSTREAM_REMARK_PARSER_H
+
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Remarks/BitstreamRemarkParser.h"
+#include "llvm/Remarks/RemarkFormat.h"
+#include "llvm/Remarks/RemarkParser.h"
+#include "llvm/Support/raw_ostream.h"
+#include <memory>
+#include <string>
+
+namespace llvm {
+namespace remarks {
+/// Parses and holds the state of the latest parsed remark.
+struct BitstreamRemarkParser : public RemarkParser {
+  /// The buffer to parse.
+  BitstreamParserHelper ParserHelper;
+  /// The string table used for parsing strings.
+  Optional<ParsedStringTable> StrTab;
+  /// Temporary remark buffer used when the remarks are stored separately.
+  std::unique_ptr<MemoryBuffer> TmpRemarkBuffer;
+  /// The common metadata used to decide how to parse the buffer.
+  /// This is filled when parsing the metadata block.
+  uint64_t ContainerVersion;
+  uint64_t RemarkVersion;
+  BitstreamRemarkContainerType ContainerType;
+  /// Wether the parser is ready to parse remarks.
+  bool ReadyToParseRemarks = false;
+
+  /// Create a parser that expects to find a string table embedded in the
+  /// stream.
+  BitstreamRemarkParser(StringRef Buf)
+      : RemarkParser(Format::Bitstream), ParserHelper(Buf) {}
+
+  /// Create a parser that uses a pre-parsed string table.
+  BitstreamRemarkParser(StringRef Buf, ParsedStringTable StrTab)
+      : RemarkParser(Format::Bitstream), ParserHelper(Buf),
+        StrTab(std::move(StrTab)) {}
+
+  Expected<std::unique_ptr<Remark>> next() override;
+
+  static bool classof(const RemarkParser *P) {
+    return P->ParserFormat == Format::Bitstream;
+  }
+
+  /// Parse and process the metadata of the buffer.
+  Error parseMeta();
+
+  /// Parse a Bitstream remark.
+  Expected<std::unique_ptr<Remark>> parseRemark();
+
+private:
+  /// Helper functions.
+  Error processCommonMeta(BitstreamMetaParserHelper &Helper);
+  Error processStandaloneMeta(BitstreamMetaParserHelper &Helper);
+  Error processSeparateRemarksFileMeta(BitstreamMetaParserHelper &Helper);
+  Error processSeparateRemarksMetaMeta(BitstreamMetaParserHelper &Helper);
+  Expected<std::unique_ptr<Remark>>
+  processRemark(BitstreamRemarkParserHelper &Helper);
+  Error processExternalFilePath(Optional<StringRef> ExternalFilePath);
+};
+
+Expected<std::unique_ptr<BitstreamRemarkParser>>
+createBitstreamParserFromMeta(StringRef Buf,
+                              Optional<ParsedStringTable> StrTab = None);
+
+} // end namespace remarks
+} // end namespace llvm
+
+#endif /* LLVM_LIB_REMARKS_BITSTREAM_REMARK_PARSER_H */