Add vfs support to toolchain. (#2888)

This adds vfs support to the toolchain, allowing Driver to take in-memory inputs in tests. As a consequence, I'm simplifying SourceBuffer: rather than allowing tests to pass in their own memory buffer, I'm using InMemoryFileSystem to push for greater consistency with production code. This does hit a quirk where I need to be careful about null terminator handling because fuzzer imports don't always have one, but that's probably more robust anyways.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parser/parse_tree_fuzzer.cpp

@@ -22,9 +22,14 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
   if (size > 100000) {
     return 0;
   }
-
-  auto source = SourceBuffer::CreateFromText(
-      llvm::StringRef(reinterpret_cast<const char*>(data), size));
+  static constexpr llvm::StringLiteral TestFileName = "test.carbon";
+  llvm::vfs::InMemoryFileSystem fs;
+  llvm::StringRef data_ref(reinterpret_cast<const char*>(data), size);
+  CARBON_CHECK(fs.addFile(
+      TestFileName, /*ModificationTime=*/0,
+      llvm::MemoryBuffer::getMemBuffer(data_ref, /*BufferName=*/TestFileName,
+                                       /*RequiresNullTerminator=*/false)));
+  auto source = SourceBuffer::CreateFromFile(fs, TestFileName);
 
   // Lex the input.
   auto tokens = TokenizedBuffer::Lex(*source, NullDiagnosticConsumer());