reimplement the regex matching strategy by building a single

regex and matching it instead of trying to match chunks at a time.
Matching chunks at a time broke with check lines like 
  CHECK: foo {{.*}}bar
because the .* would eat the entire rest of the line and bar would
never match.

Now we just escape the fixed strings for the user, so that something
like:
  CHECK: a() {{.*}}???
is matched as:
  CHECK: {{a\(\) .*\?\?\?}}
transparently "under the covers".

llvm-svn: 82779