William Fisher:  A bug in __lookahead::exec causes /(?=^)b/ to match ab. When  makes a recursive call to , it passes true for the value of . This causes a beginning-of-line anchor (^) inside a lookahead assertion to match anywhere in the text.  This fixes http://llvm.org/bugs/show_bug.cgi?id=11118

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185196 91177308-0d34-0410-b5e6-96231b3b80d8