common: Decouple LLVM hashing dependencies from common/hashing.h (#7646)

- Move formatted printing to `hashing.cpp` instead of `hashing.h`
- Separate APInt and APFloat hashing specializations into a new
`hashing_llvm.h`
- Update toolchain/base dependencies and include sites that hash LLVM
data types to include `hashing_llvm.h`

Combined, this reduces the transitive includes caused by `hashing.h`.

Assisted-by: Antigravity with Gemini

common/hashing.cpp

@@ -6,8 +6,14 @@
 
 #include <cstddef>
 
+#include "llvm/Support/FormatVariadic.h"
+
 namespace Carbon {
 
+auto HashCode::Print(llvm::raw_ostream& out) const -> void {
+  out << llvm::formatv("{0:x16}", value_);
+}
+
 auto Hasher::HashSizedBytesLarge(llvm::ArrayRef<std::byte> bytes) -> void {
   const std::byte* data_ptr = bytes.data();
   const ssize_t size = bytes.size();