[llvm][clang][modules] Fix test failure on big-endian bots

After 6fb08d8f,`clang/test/Modules/ModuleDebugInfoDwoId.cpp` started failing on a number of big-endian build bots (clang-ppc64be-linux-multistage, clang-ppc64be-linux-test-suite). This patch attempts to fix that by creating an API on `llvm::BitstreamWriter` that allows backpatching individual bytes. This API is then used from `clang::ASTWriter` to avoid endianness mismatch.