Add a String literal that returns a String

Add a String literal that returns a String
https://bugs.webkit.org/show_bug.cgi?id=191425
<rdar://problem/45914556>

Reviewed by Sam Weinig.

Source/WebCore:

Use _str where possible.

API Test in WPT.

* Modules/fetch/FetchRequest.cpp:
(WebCore::computeReferrer):
* Modules/indexeddb/IDBKeyPath.cpp:
(WebCore::loggingString):
* Modules/webdatabase/OriginLock.cpp:
(WebCore::OriginLock::lockFileNameForPath):
* css/CSSBasicShapes.cpp:
(WebCore::updateCornerRadiusWidthAndHeight):
* html/canvas/WebGL2RenderingContext.cpp:
(WebCore::WebGL2RenderingContext::getParameter):
* html/canvas/WebGLRenderingContext.cpp:
(WebCore::WebGLRenderingContext::getParameter):
* loader/LinkHeader.cpp:
(WebCore::parseParameterValue):
* loader/LinkLoader.cpp:
(WebCore::LinkLoader::preloadIfNeeded):
* page/NavigatorBase.cpp:
(WebCore::NavigatorBase::platform):
* platform/DateComponents.cpp:
(WebCore::DateComponents::toString const):
* platform/mac/PlatformEventFactoryMac.mm:
(WebCore::keyIdentifierForKeyEvent):
* rendering/RenderListMarker.cpp:
(WebCore::RenderListMarker::suffix const):
* rendering/RenderMenuList.cpp:
(RenderMenuList::setText):
* testing/InternalSettings.cpp:
(WebCore::InternalSettings::userInterfaceDirectionPolicy):
(WebCore::InternalSettings::systemLayoutDirection):
* testing/Internals.cpp:
(WebCore::Internals::shadowRootType const):
(WebCore::Internals::getCurrentCursorInfo):

Source/WTF:

Add a new String literal, _str, that will return a String type.
This is useful when ""_s won't work, such as for things that
don't take an ASCIILiteral directly e.g. ExceptionOr<String>
or Variants.

* wtf/text/WTFString.h:
(WTF::StringLiterals::operator _str): Added.

Tools:

Test _str.

* TestWebKitAPI/Tests/WTF/WTFString.cpp:
(TestWebKitAPI::TEST):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@237990 268f45cc-cd09-0410-ab3c-d52691b4dbfc