Add the concept of "modifiers" to the clang diagnostic format

strings.  This allows us to have considerable flexibility in how
these things are displayed and provides extra information that
allows us to merge away diagnostics that are very similar.

Diagnostic modifiers are a string of characters with the regex
[-a-z]+ that occur between the % and digit.  They may 
optionally have an argument that can parameterize them.

For now, I've added two example modifiers.  One is a very useful
tool that allows you to factor commonality across diagnostics
that need single words or phrases combined.  Basically you can
use %select{a|b|c}4 with with an integer argument that selects
either a/b/c based on an integer value in the range [0..3).

The second modifier is also an integer modifier, aimed to help
English diagnostics handle plurality.  "%s3" prints to 's' if 
integer argument #3 is not 1, otherwise it prints to nothing.
I'm fully aware that 's' is an English concept and doesn't
apply to all situations (mouse vs mice).  However, this is very
useful and we can add other crazy modifiers once we add support
for polish! ;-)

I converted a couple C++ diagnostics over to use this as an
example, I'd appreciate it if others could merge the other
likely candiates.  If you have other modifiers that you want,
lets talk on cfe-dev.

llvm-svn: 59803