No need to export global instances of folly::AsciiCase(Ins|S)ensitive
No need to export global instances of folly::AsciiCase(Ins|S)ensitive

Summary: [Folly] No need to export global instances of `folly::AsciiCaseInsensitive` and `folly::AsciiCaseSensitive`.

Calling code may simply construct instances at will - the compiler will optimize away the object construction since it has only the default ctor/dtor, no storage, and no vtable.

Test Plan: CI.

Reviewed By: @fugalh

Differential Revision: D2437419
zhlong73/mcrouter,nvaller/mcrouter,nvaller/mcrouter,nvaller/mcrouter,facebook/mcrouter,yqzhang/mcrouter,reddit/mcrouter,is00hcw/mcrouter,reddit/mcrouter,zhlong73/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,facebook/mcrouter,synecdoche/mcrouter,facebook/mcrouter,synecdoche/mcrouter,synecdoche/mcrouter,is00hcw/mcrouter,yqzhang/mcrouter,nvaller/mcrouter,reddit/mcrouter,reddit/mcrouter,zhlong73/mcrouter,is00hcw/mcrouter,facebook/mcrouter,is00hcw/mcrouter,zhlong73/mcrouter,yqzhang/mcrouter