[LLD][COFF] Merge .bss at end of .data section.

Because it is full of zeros, it is expected that as much of it as
possible is elided from the actual image, and that cannot happen if
there is initialized data in the section after it.

Author: jeremyd2019

lld/test/COFF/merge-data-bss.test

@@ -0,0 +1,92 @@
+# RUN: yaml2obj %s -o %t.obj
+# RUN: lld-link /out:%t.exe /entry:main /subsystem:console /force \
+# RUN:   /merge:.other=.data %t.obj /debug
+# RUN: llvm-readobj --sections %t.exe | FileCheck %s
+
+# CHECK:      Name: .data
+# CHECK-NEXT: VirtualSize: 0x2018
+# CHECK-NEXT: VirtualAddress: 0x3000
+# CHECK-NEXT: RawDataSize: 512
+# CHECK-NOT: Name: .other
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     '90'
+    SizeOfRawData:   1
+  - Name:            .data
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     '010000000000000002'
+    SizeOfRawData:   9
+  - Name:            .bss
+    Characteristics: [ IMAGE_SCN_CNT_UNINITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     ''
+    SizeOfRawData:   8192
+  - Name:            .other
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     '030000000000000004'
+    SizeOfRawData:   9
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          1
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        4027552580
+      Number:          1
+  - Name:            .data
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          9
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        4185224559
+      Number:          2
+  - Name:            .bss
+    Value:           0
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          8192
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          3
+  - Name:            .other
+    Value:           0
+    SectionNumber:   4
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          9
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        1054931164
+      Number:          4
+  - Name:            main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...