[MC] Handle unknown literal register numbers in .cfi_* directives

r230670 introduced a step to map EH register numbers to standard
DWARF register numbers. This failed to consider the case when a
user .cfi_* directive uses an integer literal rather than a
register name, to specify a DWARF register number that has no
corresponding LLVM register number (e.g. a special register that
the compiler and assembler have no name for).

Fixes PR34028.

Patch by Roland McGrath

Differential Revision: https://reviews.llvm.org/D36493

llvm-svn: 319586