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.

source/source_buffer.h

@@ -85,7 +85,9 @@ class SourceBuffer {
   };
 
   explicit SourceBuffer(llvm::StringRef fake_filename, std::string buffer_text)
-      : filename_(fake_filename.str()), is_string_rep_(true), string_storage_(buffer_text) {
+      : filename_(fake_filename.str()),
+        is_string_rep_(true),
+        string_storage_(buffer_text) {
     text_ = string_storage_;
   }