[Driver] Drop $DEFAULT_TRIPLE-$name as a fallback program name

[MaskRay]

Summary

D13340 introduced this behavior which is not needed even for mips.
This was raised on https://lists.llvm.org/pipermail/cfe-dev/2020-May/065437.html
but no action was taken.

This was raised again in https://lists.llvm.org/pipermail/cfe-dev/2021-April/067974.html
"The LLVM host/target TRIPLE padding drama on Debian"
as it caused confusion. This patch drops the behavior.

[sylvestre.ledru]

To understand better, we will be fixing a bug from 2015, correct?

[atanasyan]

LGTM

AFAIK the MIPS LLVM-based toolchain mentioned in D13340 have not been implemented.

[arichardson]

I believe adding the default triple is causing the wrong linker to be selected for me when compiling with -m32, so I'd be happy for this code to be removed.

[nickdesaulniers]

I don't foresee this affecting Linux kernel builds.

[srhines]

Adding Dan as an FYI. While this doesn't impact Android platform or regular NDK users, I suppose that someone could have some esoteric build rules that are relying on this, but they should probably be more explicit about what they're running in those cases anyways.

[MaskRay]

In D99996#2673276, @sylvestre.ledru wrote:

To understand better, we will be fixing a bug from 2015, correct?

Yes.

In D99996#2674407, @arichardson wrote:

I believe adding the default triple is causing the wrong linker to be selected for me when compiling with -m32, so I'd be happy for this code to be removed.

Yes...

In D99996#2674495, @srhines wrote:

Adding Dan as an FYI. While this doesn't impact Android platform or regular NDK users, I suppose that someone could have some esoteric build rules that are relying on this, but they should probably be more explicit about what they're running in those cases anyways.

The 2015 behavior is that a --target=aarch64-linux-gnu cross compile may pick up x86_64-unknown-linux-gnu-{as,ld,gcc}. First most Linux programs should be named x86_64-linux-gnu-* (x86_64-pc-linux-gnu-* is more common but one needs to configure llvm-project with LLVM_DEFAULT_TARGET_TRIPLE=x86_64-pc-linux-gnu, the default is unknown).
Second x86_64-unknown-linux-gnu-{as,ld,gcc}, even if exists, is unlikely to handle aarch64-linux-gnu builds... So the case should be very rare.

clang/lib/Driver/Driver.cpp

 }
 
 void Driver::generatePrefixedToolNames(
     StringRef Tool, const ToolChain &TC,
     SmallVectorImpl<std::string> &Names) const {
   // FIXME: Needs a better variable than TargetTriple
   Names.emplace_back((TargetTriple + "-" + Tool).str());
   Names.emplace_back(Tool);
-
-  // Allow the discovery of tools prefixed with LLVM's default target triple.
-  std::string DefaultTargetTriple = llvm::sys::getDefaultTargetTriple();
-  if (DefaultTargetTriple != TargetTriple)
-    Names.emplace_back((DefaultTargetTriple + "-" + Tool).str());
 }
 
 static bool ScanDirForExecutable(SmallString<128> &Dir, StringRef Name) {
   llvm::sys::path::append(Dir, Name);
   if (llvm::sys::fs::can_execute(Twine(Dir)))
     return true;
   llvm::sys::path::remove_filename(Dir);
   return false;

clang/test/Driver/program-path-priority.c

 /// Don't create symlinks on Windows
 // UNSUPPORTED: system-windows
 
 /// Check the priority used when searching for tools
 /// Names and locations are usually in this order:
-/// <triple>-tool, tool, <default triple>-tool
-/// program path, PATH
+/// <triple>-tool, tool, program path, PATH
 /// (from highest to lowest priority)
 /// A higher priority name found in a lower priority
 /// location will win over a lower priority name in a
 /// higher priority location.
 /// Prefix dirs (added with -B) override the location,
 /// so only name priority is accounted for, unless we fail to find
 /// anything at all in the prefix.
 

 
 /// plain gcc on PATH beats default triple in program path
 // RUN: rm %t/env/notreal-none-elf-gcc
 // RUN: env "PATH=%t/env/" %t/clang -### -target notreal-none-elf %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=DEFAULT_TRIPLE_NO_NOTREAL %s
 // DEFAULT_TRIPLE_NO_NOTREAL: env/gcc"
 // DEFAULT_TRIPLE_NO_NOTREAL-NOT: -gcc"
 
-/// default triple only chosen when no others are present
+/// Pick "gcc" as a fallback. Don't pick $DEFAULT_TRIPLE-gcc.
 // RUN: rm %t/env/gcc
 // RUN: env "PATH=%t/env/" %t/clang -### -target notreal-none-elf %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=DEFAULT_TRIPLE_NO_OTHERS %s
-// DEFAULT_TRIPLE_NO_OTHERS: -gcc"
-// DEFAULT_TRIPLE_NO_OTHERS-NOT: notreal-none-elf-gcc"
-// DEFAULT_TRIPLE_NO_OTHERS-NOT: /gcc"
+// DEFAULT_TRIPLE_NO_OTHERS: "gcc"
 
 /// -B paths are searched separately so default triple will win
 /// if put in one of those even if other paths have higher priority names
 // RUN: mkdir -p %t/prefix
 /// One of these will fail when $DEFAULT_TRIPLE == %target_triple
 // RUN: test -f %t/$DEFAULT_TRIPLE-gcc && \
 // RUN:   mv %t/$DEFAULT_TRIPLE-gcc %t/prefix || true
 // RUN: test -f %t/%target_triple-gcc && \