[Impeller] Generate a Metal library symbols file for shader debugging. (#39532)

Towards fixing #120326. This allows you
to specify the sources for the Impeller Metal Library. The user is meant to
navigate to `out/<engine_variant>/shaders/entity.metallib` when Xcode prompts
for sources.

Shader debugging with line by line profiling is still not a turnkey operation
though. This works fine on macOS. But on iOS, I had to bump up the min iOS
version level to iOS 12 at least and specify `-g` to the `metal` compiler for
the sources to show up with profiling. The performance of the shaders was
identical after the changes though so I suspect engine developers will patch in
these flags locally to develop and debug shaders.

Author: chinmaygarde

impeller/renderer/backend/metal/context_mtl.mm

@@ -149,6 +149,9 @@
                      << shader_library_error.localizedDescription.UTF8String;
       return nil;
     }
+    if (!label.empty()) {
+      library.label = @(label.c_str());
+    }
     [found_libraries addObject:library];
   }
   return found_libraries;

impeller/tools/build_metal_library.py

@@ -81,6 +81,8 @@ def main():
       '-Oz',
       # Allow aggressive, lossy floating-point optimizations.
       '-ffast-math',
+      # Record symbols in a separate *.metallibsym file.
+      '-frecord-sources=flat',
       '-MF',
       args.depfile,
       '-o',
@@ -97,7 +99,7 @@ def main():
   elif args.platform == 'ios':
     command += [
         '--std=ios-metal1.2',
-        '-mios-version-min=10.0',
+        '-mios-version-min=11.0',
     ]
   elif args.platform == 'ios-simulator':
     command += [

impeller/tools/impeller.gni

@@ -130,8 +130,13 @@ template("metal_library") {
     inputs = invoker.sources
 
     metal_library_path = "$root_out_dir/shaders/$metal_library_name.metallib"
+    metal_library_symbols_path =
+        "$root_out_dir/shaders/$metal_library_name.metallibsym"
 
-    outputs = [ metal_library_path ]
+    outputs = [
+      metal_library_path,
+      metal_library_symbols_path,
+    ]
 
     script = "//flutter/impeller/tools/build_metal_library.py"