obj2yaml: Correctly round-trip default alignment.

[pcc]

Summary

Previously we were emitting the "cooked" alignment, which made it hard
to distinguish between that and the default alignment.

[hans]

lgtm

llvm/test/ObjectYAML/COFF/default-alignment.yaml

+# RUN: yaml2obj %s | obj2yaml | FileCheck %s
+
+# CHECK:      Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+# CHECK-NEXT: SectionData:     '01'
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: []
+sections:
+  - Name:            .rdata
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    SectionData:     01
+symbols:
+...

llvm/tools/obj2yaml/coff2yaml.cpp

for (const auto &ObjSection : Obj.sections()) {
     NewYAMLSection.Header.NumberOfRelocations =
         COFFSection->NumberOfRelocations;
     NewYAMLSection.Header.PointerToLineNumbers =
         COFFSection->PointerToLinenumbers;
     NewYAMLSection.Header.PointerToRawData = COFFSection->PointerToRawData;
     NewYAMLSection.Header.PointerToRelocations =
         COFFSection->PointerToRelocations;
     NewYAMLSection.Header.SizeOfRawData = COFFSection->SizeOfRawData;
-    NewYAMLSection.Alignment = ObjSection.getAlignment();
+    uint32_t Shift = (COFFSection->Characteristics >> 20) & 0xF;
+    NewYAMLSection.Alignment = (1U << Shift) >> 1;
     assert(NewYAMLSection.Alignment <= 8192);
 
     ArrayRef<uint8_t> sectionData;
     if (!ObjSection.isBSS())
       Obj.getSectionContents(COFFSection, sectionData);
     NewYAMLSection.SectionData = yaml::BinaryRef(sectionData);
 
     if (NewYAMLSection.Name == ".debug$S")