Change semantics of regex expectations in the diagnostic verifier

Previously, a line like

  // expected-error-re {{foo}}

treats the entirety of foo as a regex. This is inconvenient when matching type
names containing regex characters. For example, to match
"void *(class test8::A::*)(void)" inside such a regex, one would have to type
"void \*\(class test8::A::\*\)\(void\)".

This patch changes the semantics of expected-error-re to only treat the parts
of the directive wrapped in double curly braces as regexes. This avoids the
escaping problem and leads to nicer patterns for those cases; see e.g. the
change to test/Sema/format-strings-scanf.c.

(The balanced search for closing }} of a directive also makes us handle the
full directive in test\SemaCXX\constexpr-printing.cpp:41 and :53.)

Differential Revision: http://llvm-reviews.chandlerc.com/D2388