Changed the bool conversion operator on ConstString
to be explicit, to prevent horrid things like

std::string a = ConstString("foo")

from taking the path ConstString -> bool -> char
-> std::string.

This fixes, among other things, ClangFunction.

<rdar://problem/15137989>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@191934 91177308-0d34-0410-b5e6-96231b3b80d8