Fix a couple of corner cases in NameMatches

Summary:
I originally set out to move the NameMatches closer to the relevant
function and add some unit tests. However, in the process I've found a
couple of bugs in the implementation:
- the early exits where not always correct:
  - (test==pattern) does not mean the match will always suceed because
    of regular expressions
  - pattern.empty() does not mean the match will fail because the "" is
    a valid prefix of any string

So I cleaned up those and added some tests. The only tricky part here
was that regcomp() implementation on darwin did not recognise the empty
string as a regular expression and returned an REG_EMPTY error instead.
The simples fix here seemed to be to replace the empty expression with
an equivalent non-empty one.

Reviewers: clayborg, zturner

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D30094

llvm-svn: 295651