Add opt-viewer testing

Detects whether we have the Python modules (pygments, yaml) required by
opt-viewer and hooks this up to REQUIRES.

This fixes https://bugs.llvm.org/show_bug.cgi?id=34129 (the lack of opt-viewer
testing).

It's also related to https://github.com/apple/swift/pull/12938 and the idea is
to expose LLVM_HAVE_OPT_VIEWER_MODULES to the Swift cmake.

Differential Revision: https://reviews.llvm.org/D40202

Fixes since the first commit:
1. Disable syntax highlighting as different versions of pygments generate
different HTML
2. Use llvm-cxxfilt from the build

llvm-svn: 319324
diff --git a/llvm/tools/llvm-cxxfilt/llvm-cxxfilt.cpp b/llvm/tools/llvm-cxxfilt/llvm-cxxfilt.cpp
index 13024fb..9c6a161 100644
--- a/llvm/tools/llvm-cxxfilt/llvm-cxxfilt.cpp
+++ b/llvm/tools/llvm-cxxfilt/llvm-cxxfilt.cpp
@@ -75,6 +75,7 @@
   }
 
   OS << (Undecorated ? Undecorated : Mangled) << '\n';
+  OS.flush();
 
   free(Undecorated);
 }