Revert "Using an invalid -O falls back on -O3 instead of an error"

Trying to fix test failures since earlier today.

One of the tests added in this commit is outputting test/Driver/clang_f_opts.s
which the builders that build in-tree (eg. clang-native-arm-cortex-a9) are
trying to run as a test case, causing failures.

clang_f_opts.c:
  If -### doesn't emit the warning then this test probably shouldn't be in
  here in the first place. Frontend maybe?

invalid-o-level.c:
  Running %clang_cc1 in the Driver tests doesn't make sense because -cc1
  bypasses the driver. (I'm not reverting the commit that introduced this but
  please fix instead of keeping it this way.)

Reverting to fix the build failures and also so that the tests can be thought
out more thoroughly.

This reverts commit r194817.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194845 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index f5079dd..581d56d 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -299,14 +299,14 @@
   using namespace options;
   bool Success = true;
 
-  Opts.OptimizationLevel = getOptimizationLevel(Args, IK, Diags);
-  unsigned MaxOptLevel = 3;
-  if (Opts.OptimizationLevel > MaxOptLevel) {
-    // If the optimization level is not supported, fall back on the default optimization
-    Diags.Report(diag::warn_drv_optimization_value)
-        << Args.getLastArg(OPT_O)->getAsString(Args) << "-O" << MaxOptLevel;
-    Opts.OptimizationLevel = MaxOptLevel;
+  unsigned OptLevel = getOptimizationLevel(Args, IK, Diags);
+  if (OptLevel > 3) {
+    Diags.Report(diag::err_drv_invalid_value)
+      << Args.getLastArg(OPT_O)->getAsString(Args) << OptLevel;
+    OptLevel = 3;
+    Success = false;
   }
+  Opts.OptimizationLevel = OptLevel;
 
   // We must always run at least the always inlining pass.
   Opts.setInlining(