MachO,test: Test DWARF section's begin symbol

MCObjectFileInfo::initMachOMCObjectFileInfo creates DWARF sections with
a temporary label as the `Begin` symbol, different from other object
file formats' section symbol.

 #150574 caused a regression that removed the label for MCAsmStreamer,
which was caught by no test but a ystem-darwin/target-aarch64 specific diagnostics-dsym.test

Author: MaskRay

llvm/test/DebugInfo/X86/stringpool.ll

@@ -28,8 +28,12 @@ source_filename = "test/DebugInfo/X86/stringpool.ll"
 ; LINUX: .section .debug_str,"MS",@progbits,1
 ; LINUX: yyyy
 
+; DARWIN:      .section __DWARF,__debug_abbrev,regular,debug
+; DARWIN-NEXT: Lsection_abbrev:
+
 ; Verify that we refer to 'yyyy' with a direct offset.
 ; DARWIN: .section        __DWARF,__debug_info,regular,debug
+; DARWIN: Lset1 = Lsection_abbrev-Lsection_abbrev
 ; DARWIN: DW_TAG_variable
 ; DARWIN:             .long   [[YYYY:[0-9]+]]
 ; DARWIN-NEXT:        .long   {{[0-9]+}}              ## DW_AT_type