Plumbing for a Virtual File System

Adds the -vfsoverlay frontend option that enables the user to pass
VFS overlay YAML files to Swift. These files define a (potentially
many-layered) virtual mapping on which we predicate a VFS.

Switch all input-based memory buffer reads in the Frontend to the new
FileSystem-based approach.

lib/Basic/FileSystem.cpp

@@ -13,6 +13,7 @@
 #include "swift/Basic/FileSystem.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Process.h"
+#include "clang/Basic/FileManager.h"
 
 using namespace swift;
 
@@ -88,3 +89,18 @@ std::error_code swift::moveFileIfDifferent(const llvm::Twine &source,
   // If we get here, we weren't able to prove that the files are the same.
   return fs::rename(source, destination);
 }
+
+llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
+swift::vfs::getFileOrSTDIN(clang::vfs::FileSystem &FS,
+                           const llvm::Twine &Filename,
+                           int64_t FileSize,
+                           bool RequiresNullTerminator,
+                           bool IsVolatile) {
+  llvm::SmallString<256> NameBuf;
+  llvm::StringRef NameRef = Filename.toStringRef(NameBuf);
+
+  if (NameRef == "-")
+    return llvm::MemoryBuffer::getSTDIN();
+  return FS.getBufferForFile(Filename, FileSize,
+                             RequiresNullTerminator, IsVolatile);
+}