Parse 'technical term' format specifier.

Objective-C format strings now support modifier flags
that can be attached to a '@' conversion.  Currently
the only one supported, as of iOS 9 and OS X 10.11,
is the new "technical term", denoted by the flag "tt",
for example:

  %[tt]@

instead of just:

  %@

The 'tt' stands for "technical term", which is used
by the string-localization facilities on Darwin to
add the appropriate spacing or quotation depending
the language locale.

Implements <rdar://problem/20374720>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241243 91177308-0d34-0410-b5e6-96231b3b80d8