CI: point LLVM's DIA SDK at the installed Visual Studio; semantic index for the language server

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/index.h

@@ -0,0 +1,39 @@
+#pragma once
+// What the compiler learns about a program while checking it, for the language server:
+// what each name is, its type, and where it was defined.
+
+#include <string>
+#include <vector>
+
+struct SymRef {  // a name in the source
+    int file, line, col, len;
+    std::string hover;          // markdown shown on mouse-over
+    int defFile = -1, defLine = 0, defCol = 0;
+};
+
+struct VarInfo {  // a variable or parameter, for completion
+    std::string name, type;
+    int file, line, col;
+    int fnFile, fnLine, fnEnd;  // the function it lives in
+};
+
+struct FnInfo {
+    std::string module, name, signature, doc;
+    std::vector<std::string> params;
+    int file, line, col, endLine;
+};
+
+struct StructIndex {
+    std::string module, name;
+    std::vector<std::pair<std::string, std::string>> fields;  // name, type
+    int file, line, col;
+};
+
+struct SemIndex {
+    std::vector<SymRef> refs;
+    std::vector<VarInfo> vars;
+    std::vector<FnInfo> fns;
+    std::vector<StructIndex> structs;
+};
+
+inline SemIndex *g_index = nullptr;  // set by the language server; null when compiling