Introduce StringImpl::StaticStringImpl with constexpr constructor

Introduce StringImpl::StaticStringImpl with constexpr constructor
https://bugs.webkit.org/show_bug.cgi?id=165093

Reviewed by Darin Adler.

Source/WTF:

This patch adds new class, StringImpl::StaticStringImpl.
By using this class, we can easily create static StringImpls.
This class has constexpr constructor. You can initialize instances
of this class as global static variables without invoking global
constructors.

We already have similar system, StaticASCIILiteral. But using it
requires some special perl script since we need to calculate
hash value. On the other hand, we can use StaticStringImpl without
any script supports since we implement constexpr hash function.
In the future, we will replace all the use of StaticASCIILiteral
with this StaticStringImpl.

We define empty / null strings as StaticStringImpl. And we make
StringImpl::empty() & StringImpl::null() inline functions.

* wtf/Hasher.h:
(WTF::StringHasher::hashWithTop8BitsMasked):
(WTF::StringHasher::hash):
(WTF::StringHasher::finalize):
(WTF::StringHasher::finalizeAndMaskTop8Bits):
(WTF::StringHasher::computeLiteralHash):
(WTF::StringHasher::computeLiteralHashAndMaskTop8Bits):
(WTF::StringHasher::avalancheBits3):
(WTF::StringHasher::avalancheBits2):
(WTF::StringHasher::avalancheBits1):
(WTF::StringHasher::avalancheBits0):
(WTF::StringHasher::avalancheBits):
(WTF::StringHasher::avoidZero):
(WTF::StringHasher::processPendingCharacter):
(WTF::StringHasher::calculateWithRemainingLastCharacter1):
(WTF::StringHasher::calculateWithRemainingLastCharacter0):
(WTF::StringHasher::calculateWithRemainingLastCharacter):
(WTF::StringHasher::calculate1):
(WTF::StringHasher::calculate0):
(WTF::StringHasher::calculate):
(WTF::StringHasher::computeLiteralHashImpl):
* wtf/text/StringImpl.cpp:
* wtf/text/StringImpl.h:
(WTF::StringImpl::StaticStringImpl::StaticStringImpl):
(WTF::StringImpl::null):
(WTF::StringImpl::empty):
* wtf/text/StringStatics.cpp:
(WTF::StringImpl::null): Deleted.
(WTF::StringImpl::empty): Deleted.

Tools:

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

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@209179 268f45cc-cd09-0410-ab3c-d52691b4dbfc