Add an c++ itanium demangler to llvm.

This adds a copy of the demangler in libcxxabi.

The code also has no dependencies on anything else in LLVM. To enforce
that I added it as another library. That way a BUILD_SHARED_LIBS will
fail if anyone adds an use of StringRef for example.

The no llvm dependency combined with the fact that this has to build
on linux, OS X and Windows required a few changes to the code. In
particular:

    No constexpr.
    No alignas

On OS X at least this library has only one global symbol:
__ZN4llvm16itanium_demangleEPKcPcPmPi

My current plan is:

    Commit something like this
    Change lld to use it
    Change lldb to use it as the fallback

    Add a few #ifdefs so that exactly the same file can be used in
    libcxxabi to export abi::__cxa_demangle.

Once the fast demangler in lldb can handle any names this
implementation can be replaced with it and we will have the one true
demangler.

llvm-svn: 280732
diff --git a/llvm/tools/llvm-objdump/CMakeLists.txt b/llvm/tools/llvm-objdump/CMakeLists.txt
index 73868d8..f1239a2 100644
--- a/llvm/tools/llvm-objdump/CMakeLists.txt
+++ b/llvm/tools/llvm-objdump/CMakeLists.txt
@@ -6,6 +6,7 @@
   CodeGen
   DebugInfoDWARF
   DebugInfoPDB
+  Demangle
   MC
   MCDisassembler
   Object