Add some friendly messages to libcxx calls to abort().

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@186951 91177308-0d34-0410-b5e6-96231b3b80d8
Bill Fisher: This patch fixes a bug where std::regex in ECMAScript mode was ignoring capture groups inside lookahead assertions. 

For example, matching /(?=(a))(a)/ to "a" should yield two captures: \1 = "a", \2 = "a"

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@186954 91177308-0d34-0410-b5e6-96231b3b80d8