Treat -Wformat=0 as an alias for -Wformat.

Fixes PR9195.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148300 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/Warnings.cpp b/lib/Frontend/Warnings.cpp
index 6f44111..576dd3d 100644
--- a/lib/Frontend/Warnings.cpp
+++ b/lib/Frontend/Warnings.cpp
@@ -83,6 +83,10 @@
     for (unsigned i = 0, e = Opts.Warnings.size(); i != e; ++i) {
       StringRef Opt = Opts.Warnings[i];
 
+      // Treat -Wformat=0 as an alias for -Wno-format.
+      if (Opt == "format=0")
+        Opt = "no-format";
+
       // Check to see if this warning starts with "no-", if so, this is a
       // negative form of the option.
       bool isPositive = true;