Bitstream reader: Fix undefined behavior seen after rL364464

Summary:
After rL364464 the following tests started to fail when
running the clang-doc tests with an ubsan instrumented
build of clang-doc:
    Clang Tools :: clang-doc/single-file-public.cpp
    Extra Tools Unit Tests :: clang-doc/./ClangDocTests/BitcodeTest.emitEnumInfoBitcode
    Extra Tools Unit Tests :: clang-doc/./ClangDocTests/BitcodeTest.emitMethodInfoBitcode
    Extra Tools Unit Tests :: clang-doc/./ClangDocTests/BitcodeTest.emitRecordInfoBitcode
    Extra Tools Unit Tests :: clang-doc/./ClangDocTests/SerializeTest.emitInfoWithCommentBitcode

We need to check that the read value is in range for being
casted to the llvm::bitc::FixedAbbrevIDs enum, before the
cast in ClangDocBitcodeReader::skipUntilRecordOrBlock.

SerializedDiagnosticReader::skipUntilRecordOrBlock was updated
in the same way.

Reviewers: jfb

Reviewed By: jfb

Subscribers: Bigcheese, vsapsai, bruno, ilya-biryukov, dexonsmith, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64262

llvm-svn: 365239