Improve name mangling for blocks and support mangling of static local

Improve name mangling for blocks and support mangling of static local
variables within blocks. We loosely follow GCC's mangling, but since
these are always internal symbols the names don't really matter. I
intend to revisit block mangling later, because GCC's mangling is
rather verbose. <rdar://problem/8015719>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@104610 91177308-0d34-0410-b5e6-96231b3b80d8