pw_build: Introduce config for enabling -Wshadow-all

-Wshadow-all will be added to internal_strict_warnings in a later
change.

Change-Id: I2ddaa006e200ecfabfc1ef5606e2f49fa4e071b3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/231651
Reviewed-by: Dave Roth <[email protected]>
Lint: Lint 🤖 <[email protected]>
Commit-Queue: Wyatt Hepler <[email protected]>
Pigweed-Auto-Submit: Wyatt Hepler <[email protected]>

pw_build/BUILD.gn

@@ -189,6 +189,12 @@ config("extra_strict_warnings") {
 # This config MUST NOT be used downstream to allow for warnings to be
 # added in the future without breaking downstream.
 config("internal_strict_warnings") {
+  configs = [ ":internal_strict_warnings_core" ]
+}
+
+# Core flags from ":internal_strict_warnings" that are enabled throughout
+# Pigweed. These are split out so ":shadow_all" can be disabled where needed.
+config("internal_strict_warnings_core") {
   cflags = [ "-Wswitch-enum" ]
 
   if (is_clang) {
@@ -204,6 +210,13 @@ config("internal_strict_warnings") {
   }
 }
 
+# Keep "-Wshadow-all" separate so it can be disabled individually.
+config("shadow_all") {
+  if (is_clang) {
+    cflags = [ "-Wshadow-all" ]
+  }
+}
+
 config("pedantic_warnings") {
   cflags = [
     # Enable -Wpedantic, but disable a few warnings.