Greatly reduce build warning spam.

No-one ever fixes unused-* or deprecated-* warnings in external/ (nor should they!).

-Wno-deprecated-enum-enum-conversion should probably have been added
next to -Wno-deprecated-anon-enum-enum-conversion when it was added,
since it's a major spam contributor, and equally unlikely that anyone
will clean them up.

-Wno-deprecated-dynamic-exception-spec is a weird special case because
it should be irrelevant after our next libc++ update, but in the
meantime -- because libc++ is used everywhere -- this is a massive spam
contributor that no-one on Android cares about because we don't use
exceptions (and the people seeing this can't fix libc++ anyway).

Test: local clean builds
Change-Id: I098202337e9e5026c4c5215dbf5a2abf1fbbdca8

cc/config/global.go

@@ -272,7 +272,8 @@ var (
 		"-Wno-range-loop-construct",                 // http://b/153747076
 		"-Wno-zero-as-null-pointer-constant",        // http://b/68236239
 		"-Wno-deprecated-anon-enum-enum-conversion", // http://b/153746485
-		"-Wno-pessimizing-move",                     // http://b/154270751
+		"-Wno-deprecated-enum-enum-conversion",
+		"-Wno-pessimizing-move", // http://b/154270751
 		// New warnings to be fixed after clang-r399163
 		"-Wno-non-c-typedef-for-linkage", // http://b/161304145
 		// New warnings to be fixed after clang-r428724
@@ -286,6 +287,13 @@ var (
 		// New warnings to be fixed after clang-r475365
 		"-Wno-error=single-bit-bitfield-constant-conversion", // http://b/243965903
 		"-Wno-error=enum-constexpr-conversion",               // http://b/243964282
+
+		// Irrelevant on Android because _we_ don't use exceptions, but causes
+		// lots of build noise because libcxx/libcxxabi do. This can probably
+		// go away when we're on a new enough libc++, but has to be global
+		// until then because it causes warnings in the _callers_, not the
+		// project itself.
+		"-Wno-deprecated-dynamic-exception-spec",
 	}
 
 	noOverride64GlobalCflags = []string{}
@@ -320,6 +328,9 @@ var (
 
 		// http://b/239661264
 		"-Wno-deprecated-non-prototype",
+
+		"-Wno-unused",
+		"-Wno-deprecated",
 	}
 
 	// Similar to noOverrideGlobalCflags, but applies only to third-party code