[ifs] Add --strip-needed flag

[abrachet]

Apply clang-format suggestion

[mcgrathr]

LGTM but Haowei should give the final say.

[haowei]

llvm/tools/llvm-ifs/llvm-ifs.cpp

106

Since this flag affects both IFS output as well as ELF output. I think it is better to rename this variable to StripNeededLibs

[abrachet]

• Changed cl::opt name
• Clear NeededLibs vector inline instead of using separate function

[abrachet]

llvm/tools/llvm-ifs/llvm-ifs.cpp

106

Done. Out of curiosity is there a reason that these other flags are only meaningful when outputting ifs? Why not have strip-undefined strip undefined syms when outputting elf stubs too?

[haowei]

llvm/tools/llvm-ifs/llvm-ifs.cpp

106

At the time when it was implemented, we thought it was unnecessary. So it only applied to IFS file. I guess it is OK to make the flag applies to the ELF as well.

llvm/test/tools/llvm-ifs/strip-needed.test

+## Test --strip-needed flag
+
+# RUN: llvm-ifs --input-format=IFS --strip-needed --output-ifs=- %s | FileCheck %s
+
+# RUN: llvm-ifs --input-format=IFS --output-elf=- %s | \
+# RUN: obj2yaml | FileCheck %s --check-prefix=FROM-ELF
+
+# RUN: llvm-ifs --input-format=IFS --strip-needed --output-elf=- %s | \
+# RUN: obj2yaml | FileCheck %s --check-prefix=FROM-ELF-STRIP-NEEDED
+
+--- !ifs-v1
+IfsVersion: 3.0
+Target: { ObjectFormat: ELF, Arch: AArch64, Endianness: little, BitWidth: 64 }
+NeededLibs:
+  - ''
+Symbols: []
+...
+
+# CHECK:      --- !ifs-v1
+# CHECK-NEXT: IfsVersion: {{[1-9]\d*\.(0|([1-9]\d*))}}
+# CHECK-NEXT: Target: { ObjectFormat: ELF, Arch: AArch64, Endianness: little, BitWidth: 64 }
+# CHECK-NEXT: Symbols: []
+# CHECK-NEXT: ...
+
+# FROM-ELF: DT_NEEDED
+# FROM-ELF-STRIP-NEEDED-NOT: DT_NEEDED

llvm/tools/llvm-ifs/llvm-ifs.cpp

 cl::opt<bool> StripIFSTarget(
     "strip-ifs-target",
     cl::desc("Strip all target information away from IFS output"),
     cl::cat(IfsCategory));
 cl::opt<bool>
     StripUndefined("strip-undefined",
                    cl::desc("Strip undefined symbols from IFS output"),
                    cl::cat(IfsCategory));
+cl::opt<bool> StripNeededLibs("strip-needed",
+                              cl::desc("Strip needed libs from output"),
+                              cl::cat(IfsCategory));
 
 cl::opt<std::string>
     SoName("soname",
            cl::desc("Manually set the DT_SONAME entry of any emitted files"),
            cl::value_desc("name"), cl::cat(IfsCategory));
 cl::opt<std::string> OutputFilePath("output",
                                     cl::desc("Output file **DEPRECATED**"),
                                     cl::cat(IfsCategory));
if (OptBitWidth.getNumOccurrences() == 1)
     OverrideBitWidth = OptBitWidth.getValue();
   if (OptTargetTriple.getNumOccurrences() == 1)
     OverrideTriple = OptTargetTriple.getValue();
   Error OverrideError = overrideIFSTarget(
       Stub, OverrideArch, OverrideEndianness, OverrideBitWidth, OverrideTriple);
   if (OverrideError)
     fatalError(std::move(OverrideError));
 
+  if (StripNeededLibs)
+    Stub.NeededLibs.clear();
+
   if (OutputELFFilePath.getNumOccurrences() == 0 &&
       OutputIFSFilePath.getNumOccurrences() == 0 &&
       OutputTBDFilePath.getNumOccurrences() == 0) {
     if (OutputFormat.getNumOccurrences() == 0) {
       WithColor::error() << "at least one output should be specified.";
       return -1;
     }
   } else if (OutputFormat.getNumOccurrences() == 1) {