[Mangle] Add flag to asm labels to disable '\01' prefixing

LLDB synthesizes decls using asm labels. These decls cannot have a mangle
different than the one specified in the label name. I.e., the '\01' prefix
should not be added.

Fixes an expression evaluation failure in lldb's TestVirtual.py on iOS.

rdar://45827323

Differential Revision: https://reviews.llvm.org/D67774

llvm-svn: 372903