Introduce Regex::isLiteralERE function.

This will be used to implement an optimisation for literal entries
in special case lists.

Differential Revision: http://llvm-reviews.chandlerc.com/D1278
Introduce an optimisation for special case lists with large numbers of literal entries.

Our internal regex implementation does not cope with large numbers
of anchors very efficiently.  Given a ~3600-entry special case list,
regex compilation can take on the order of seconds.  This patch solves
the problem for the special case of patterns matching literal global
names (i.e. patterns with no regex metacharacters).  Rather than
forming regexes from literal global name patterns, add them to
a StringSet which is checked before matching against the regex.
This reduces regex compilation time by an order of roughly thousands
when reading the aforementioned special case list, according to a
completely unscientific study.

No test cases.  I figure that any new tests for this code should
check that regex metacharacters are properly recognised.  However,
I could not find any documentation which documents the fact that the
syntax of global names in special case lists is based on regexes.
The extent to which regex syntax is supported in special case lists
should probably be decided on/documented before writing tests.

Differential Revision: http://llvm-reviews.chandlerc.com/D1150