Poor performance with hashing large constant strings

[clubby789]

Found as part of rust-lang/rust#103812.

When creating a very large constant blob, a large proportion of compile time is spent hashing the string with djbHash to insert it into CDSConstants

llvm-project/llvm/lib/IR/Constants.cpp

Lines 2877 to 2880 in 244331a

	auto &Slot =
	*Ty->getContext()
	.pImpl->CDSConstants.insert(std::make_pair(Elements, nullptr))
	.first;

@alloc = private unnamed_addr constant <{ [104857600 x i8] }> <{ [104857600 x i8] c"... VERY LONG DATA ..." }>, align 1

cc @pcwalton

[clubby789]

https://reviews.llvm.org/D137601