Add --build-id=fast as a synonym for --build-id.

[ruiu]

Summary

When you omit an argument, most options fall back to their defaults.
For example, --color-diagnostics is a synonym for --color-diagnostics=auto.
We don't have a way to specify the default choice for --build-id, so we
can't describe --build-id (without an argument) in that way.
This patch adds "fast" for the default build-id choice.

lld/ELF/Driver.cpp

getBuildId(opt::InputArgList &Args) {
   auto *Arg = Args.getLastArg(OPT_build_id, OPT_build_id_eq);
   if (!Arg)
     return {BuildIdKind::None, {}};
 
   if (Arg->getOption().getID() == OPT_build_id)
     return {BuildIdKind::Fast, {}};
 
   StringRef S = Arg->getValue();
+  if (S == "fast")
+    return {BuildIdKind::Fast, {}};
   if (S == "md5")
     return {BuildIdKind::Md5, {}};
   if (S == "sha1" || S == "tree")
     return {BuildIdKind::Sha1, {}};
   if (S == "uuid")
     return {BuildIdKind::Uuid, {}};
   if (S.startswith("0x"))
     return {BuildIdKind::Hexstring, parseHex(S.substr(2))};

lld/test/ELF/build-id.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 
 # RUN: ld.lld --build-id %t -o %t2
 # RUN: llvm-readobj -s %t2 | FileCheck -check-prefix=ALIGN %s
 
 # RUN: ld.lld --build-id %t -o %t2 -threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s
+# RUN: ld.lld --build-id=fast %t -o %t2 -threads
+# RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s
 # RUN: ld.lld --build-id %t -o %t2 -no-threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s
 
 # RUN: ld.lld --build-id=md5 %t -o %t2 -threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=MD5 %s
 # RUN: ld.lld --build-id=md5 %t -o %t2 -no-threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=MD5 %s