Refactory the IR/builder relationship (#2342)

I'm thinking about how to handle multiple files, and I think the current IRFactory is useful as a file-focused thing. So shifting/renaming accordingly. (doing this in its own PR to make the history a little cleaner for git's move detection)

toolchain/semantics/semantics_ir.cpp

@@ -8,9 +8,15 @@
 #include "llvm/Support/FormatVariadic.h"
 #include "toolchain/lexer/tokenized_buffer.h"
 #include "toolchain/semantics/semantics_node.h"
+#include "toolchain/semantics/semantics_parse_tree_handler.h"
 
 namespace Carbon {
 
+SemanticsIR::SemanticsIR(const TokenizedBuffer& tokens,
+                         const ParseTree& parse_tree) {
+  SemanticsParseTreeHandler(tokens, parse_tree, *this).Build();
+}
+
 auto SemanticsIR::Print(llvm::raw_ostream& out) const -> void {
   out << "identifiers = {\n";
   for (int32_t i = 0; i < static_cast<int32_t>(identifiers_.size()); ++i) {