Require braces on conditions and loops. (#218)

The rationale and rule for this was added in #194 to the C++ style guide
we are using for Carbon.

I've applied the automated fixes from running `clang-tidy` over all the
code, and then run `clang-format` afterward.

There are a few places where `clang-format` fixed a formatting issue
that snuck through in prior commits. These were rare enough that it
didn't seem worth splitting them out into a separate change.

parser/parse_tree_fuzzer.cpp

@@ -16,8 +16,9 @@ namespace Carbon {
 extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
                                       std::size_t size) {
   // We need two bytes of data to compute a file name length.
-  if (size < 2)
+  if (size < 2) {
     return 0;
+  }
   unsigned short raw_filename_length;
   std::memcpy(&raw_filename_length, data, 2);
   data += 2;
@@ -25,8 +26,9 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
   std::size_t filename_length = raw_filename_length;
 
   // We need enough data to populate this filename length.
-  if (size < filename_length)
+  if (size < filename_length) {
     return 0;
+  }
   llvm::StringRef filename(reinterpret_cast<const char*>(data),
                            filename_length);
   data += filename_length;
@@ -41,14 +43,16 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
 
   // Lex the input.
   auto tokens = TokenizedBuffer::Lex(source, emitter);
-  if (tokens.HasErrors())
+  if (tokens.HasErrors()) {
     return 0;
+  }
 
   // Now parse it into a tree. Note that parsing will (when asserts are enabled)
   // walk the entire tree to verify it so we don't have to do that here.
   ParseTree tree = ParseTree::Parse(tokens, emitter);
-  if (tree.HasErrors())
+  if (tree.HasErrors()) {
     return 0;
+  }
 
   // In the absence of parse errors, we should have exactly as many nodes as
   // tokens.