Pull request 939 in include-what-you-use: Preliminary: Use std::regex instead of llvm::Regex

[notifi...@include-what-you-use.org]

New pull request 939 by kimgr: Preliminary: Use std::regex instead of llvm::Regex
https://github.com/include-what-you-use/include-what-you-use/pull/939

llvm::Regex does not support negative lookaround, so there was no way to do
negative assertions such as:

"@\"((?!.*/internal/).*)-inl.h\""

as part of mapping rules.

Switch to std::regex (which defaults to ECMAScript regex dialect) to allow these
constructs.