Print C-string literals in mapfile

This diff has the C-string literals printed into the mapfile in the symbol table like how ld64 does.

Here is what ld64's mapfile looks like with C-string literals:
```
# Path: out
# Arch: x86_64
# Object files:
[  0] linker synthesized
[  1] foo.o
# Sections:
# Address       Size            Segment Section
0x100003F7D     0x0000001D      __TEXT  __text
0x100003F9A     0x0000001E      __TEXT  __cstring
0x100003FB8     0x00000048      __TEXT  __unwind_info
# Symbols:
# Address       Size            File  Name
0x100003F7D     0x0000001D      [  1] _main
0x100003F9A     0x0000000E      [  1] literal string: Hello world!\n
0x100003FA8     0x00000010      [  1] literal string: Hello, it's me\n
0x100003FB8     0x00000048      [  0] compact unwind info
```

Here is what the new lld's Mach-O mapfile looks like:
```
# Path: /Users/rgr/local/llvm-project/build/Debug/tools/lld/test/MachO/Output/map-file.s.tmp/c-string-liter
al-out
# Arch: x86_64
# Object files:
[  0] linker synthesized
[  1] /Users/rgr/local/llvm-project/build/Debug/tools/lld/test/MachO/Output/map-file.s.tmp/c-string-literal
.o
# Sections:
# Address       Size            Segment Section
0x1000002E0     0x0000001D      __TEXT  __text
0x1000002FD     0x0000001D      __TEXT  __cstring
# Symbols:
# Address           File  Name
0x1000002E0     [  1] _main
0x1000002FD     [  1] literal string: Hello world!\n
0x10000030B     [  1] literal string: Hello, it's me\n
```

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D118077