Switch CARBON_VLOG to support a format string API. (#4283)

The goal is to replace our stream operator APIs with format string APIs
that can be made to have much less impact on inlining and other
optimizations of the performance critical path through the code.

Several experiments show that the most compact representation we can
arrange for is one that calls an uninlined function and passes a minimal
number of arguments to it. It doesn't help to do any work to minimize
the arguments such as building a lambda -- the cost of extra code to
merge the arguments is likely to outweigh the benefit.

Initial experiments showed that switching a hot but uninlined function
to this new API enabled inlining and the subsequent performance
improvement.

This also adds a 'TemplateString` utility that allows using a string
literal as a template parameter. This is useful to remove the format
string itself from the arguments passed to the function by passing it as
a template argument instead.

Currently, support is left in place for both APIs because with
`CARBON_VLOG` we can detect whether or not any message was provided
expecting a format string. This should allow incrementally migrating
code to this API. I've added some test coverage in this PR, but I'll
separate out any switching of parts of the codebase over.

The goal is to eventually replace all the usages and remove the
streaming support entirely.

This PR doesn't update `CARBON_CHECK` in the same way because it is
substantially more complex to switch. I have a few experimental PRs
looking at that and will discuss how best to approach this with the
specific challenges check presents separately. But the goal is for all
of the macro-based output APIs to move to format strings rather than
streams.

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

common/vlog_test.cpp

@@ -24,7 +24,9 @@ class VLogger {
     }
   }
 
-  void VLog() { CARBON_VLOG() << "Test\n"; }
+  void VLog() { CARBON_VLOG("Test\n"); }
+  void VLogFormatArgs() { CARBON_VLOG("Test {0} {1} {2}\n", 1, 2, 3); }
+  void VLogStream() { CARBON_VLOG() << "Test\n"; }
 
   auto TakeStr() -> std::string { return buffer_.TakeStr(); }
 
@@ -38,6 +40,10 @@ TEST(VLogTest, Enabled) {
   VLogger vlog(/*enable=*/true);
   vlog.VLog();
   EXPECT_THAT(vlog.TakeStr(), StrEq("Test\n"));
+  vlog.VLogFormatArgs();
+  EXPECT_THAT(vlog.TakeStr(), StrEq("Test 1 2 3\n"));
+  vlog.VLogStream();
+  EXPECT_THAT(vlog.TakeStr(), StrEq("Test\n"));
 }
 
 TEST(VLogTest, Disabled) {