Hide gnu extension warnings in log/log.h

system/core/include is included in the global include path using
-isystem, which hides all warnings.  Fix warnings in log/log.h
in preparation for moving from -isystem to -I.

log/log.h uses gnu extensions for zero-argument variadic macro token
pasting, which clang warns about with -Weverything.  Hide the warnings
using pragmas.

Test: m -j native
Bug: 31492149
Change-Id: Ib2113f877d811a6ecc0e91c035640b506f04bd86

include/log/log.h

@@ -41,6 +41,15 @@
 extern "C" {
 #endif
 
+// This file uses ", ## __VA_ARGS__" zero-argument token pasting to
+// work around issues with debug-only syntax errors in assertions
+// that are missing format strings.  See commit
+// 19299904343daf191267564fe32e6cd5c165cd42
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wgnu-zero-variadic-macro-arguments"
+#endif
+
 // ---------------------------------------------------------------------
 
 /*
@@ -692,6 +701,10 @@ int __android_log_buf_print(int bufID, int prio, const char *tag, const char *fm
 #endif
     ;
 
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 #ifdef __cplusplus
 }
 #endif