Implement '-Weverything', which enables all warnings except those explicitly mapped to be ignored.

Currently this includes -pedantic warnings as well; we'll need to consider whether these should
be included.

This works as expected with -Werror.

Test cases were added to Sema/warn-unused-parameters.c, but they should probably be broken off into
their own test file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137910 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/Warnings.cpp b/lib/Frontend/Warnings.cpp
index 215f8f8..d24779e 100644
--- a/lib/Frontend/Warnings.cpp
+++ b/lib/Frontend/Warnings.cpp
@@ -98,6 +98,13 @@
       Mapping = isPositive ? diag::MAP_ERROR : diag::MAP_WARNING_NO_WERROR;
       Opt = Specifier;
     }
+    
+    // -Weverything is a special case as well.  It implicitly enables all
+    // warnings, including ones not explicitly in a warning group.
+    if (Opt == "everything") {
+      Diags.setEnableAllWarnings(true);
+      continue;
+    }
 
     // -Wfatal-errors is yet another special case.
     if (Opt.startswith("fatal-errors")) {