[PDB] Make streams carry their own endianness.

Before the endianness was specified on each call to read
or write of the StreamReader / StreamWriter, but in practice
it's extremely rare for streams to have data encoded in
multiple different endiannesses, so we should optimize for the
99% use case.

This makes the code cleaner and more general, but otherwise
has NFC.

llvm-svn: 296415
diff --git a/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp b/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
index 4c85825..dff5334 100644
--- a/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
+++ b/llvm/lib/DebugInfo/PDB/Native/NativeSession.cpp
@@ -45,7 +45,8 @@
     return make_error<GenericError>(generic_error_code::invalid_path);
 
   std::unique_ptr<MemoryBuffer> Buffer = std::move(*ErrorOrBuffer);
-  auto Stream = llvm::make_unique<MemoryBufferByteStream>(std::move(Buffer));
+  auto Stream = llvm::make_unique<MemoryBufferByteStream>(
+      std::move(Buffer), llvm::support::little);
 
   auto Allocator = llvm::make_unique<BumpPtrAllocator>();
   auto File = llvm::make_unique<PDBFile>(Path, std::move(Stream), *Allocator);