Get MCSymbol out of the mangling business, and move all the logic

Get MCSymbol out of the mangling business, and move all the logic
to Mangler.  Now MCSymbol just decides whether to slap quotes around
a symbol when printing it.

This also fixes some weirdness where two MCSymbols could be created
for the same symbol, if one needed to be mangled and got mangled to
the other one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93690 91177308-0d34-0410-b5e6-96231b3b80d8