Fix rdar://8139785 "implement warning on dead expression in comma operator"

As a bonus, fix the warning for || and && operators; it was emitted even if one of the operands had side effects, e.g:

x || test_logical_foo1();

emitted a bogus "expression result unused" for 'x'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107274 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/Sema/expr-comma-c89.c b/test/Sema/expr-comma-c89.c
index d0883ba..dc42702 100644
--- a/test/Sema/expr-comma-c89.c
+++ b/test/Sema/expr-comma-c89.c
@@ -11,7 +11,7 @@
 
 
 // comma does array/function promotion in c99.
-int X[sizeof(0, (foo().c)) == sizeof(char*) ? 1 : -1];
-int Y[sizeof(0, (a,b).c) == sizeof(char*) ? 1 : -1];
-int Z[sizeof(0, (a=b).c) == sizeof(char*) ? 1 : -1];
+int X[sizeof(0, (foo().c)) == sizeof(char*) ? 1 : -1]; // expected-warning {{expression result unused}}
+int Y[sizeof(0, (a,b).c) == sizeof(char*) ? 1 : -1];   // expected-warning {{expression result unused}} expected-warning {{expression result unused}}
+int Z[sizeof(0, (a=b).c) == sizeof(char*) ? 1 : -1];  // expected-warning {{expression result unused}}