Do not emit a named symbol to denote the start of the debug_frame section

When emitting a debug_frame section, it contains a named symbol.

> echo "void foo(void) {}" | clang -arch arm64  -ffreestanding -g -c -o \
/tmp/test.o -x c -
> nm /tmp/test.o -s __DWARF __debug_frame
0000000000000200 s ltmp1

There are no such symbols emitted in any of the other DWARF sections,
this is because when the __debug_frame section is created, it doesn't
get a `BeginSymName` and so it creates a named symbol, such as `ltmp1`
and emits it when we switch to the section in MCDwarf.cpp.

This patch fixes the above issue.

Differential Revision: https://reviews.llvm.org/D153484