Meta: Add the start of a gn build based on the LLVM gn build

"based on" in this context means "largely copied from"

Meta/gn/build/buildflags.gni

@@ -0,0 +1,18 @@
+declare_args() {
+  # Build for debugging. Equivalent to is_optimized=false symbol_level=2.
+  is_debug = false
+}
+
+# args that depend on other args must live in a later declare_args() block.
+declare_args() {
+  # Whether to build with optimizations.
+  is_optimized = !is_debug
+
+  if (is_debug) {
+    # Debug info symbol level. 0: No symbols; 1: Line numbers; 2: Full symbols.
+    symbol_level = 2
+  } else {
+    # Debug info symbol level. 0: No symbols; 1: Line numbers; 2: Full symbols.
+    symbol_level = 1
+  }
+}