clangd tests: use extended regex with sed

The escaped parens seem to confuse the combination of lit, cygwin
quoting, and cygwin's sed. unxutils sed in cmd.exe is fine with both
forms, so use the extended regex form that doesn't need an escaped
paren.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@374753 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed