Slightly improve the getenv("bar") linking problem

There's been a variation of the following in the code since 2005:

    if (unoptimizable_true)
      return;
    use_this_symbol_to_force_linking(); // unreachable but never removed

Way back in 00d5508496c it was the win32 call `GetCurrentProcess`
but switched to `getenv("bar")` fairly soon after in 63e504ff43. While
that pulled in fewer dependencies and made the code portable, it's a
bit of a weird construct. The environment variable used for the `getenv`
call is "bar", which is particularly weird to see fly past when you run
`ltrace` on a binary linked against LLVM.

In this patch I don't try to replace this construct wholesale - it's
still required for architectural reasons I'm not able to tackle right
now, but I did try and make it slightly less weird and opaque:

- It gives the construct a name
- The environment variable hints where this comes from and that its
  value is ignored

Combined, this should be a bit of improvement for the next person who
wonders what LLVM is up to when they trace their process or see
smatterings of `getenv("bar")` dotted around the source.

GitOrigin-RevId: f992ae4fd16357116b341a1c8291b970787dc462

include/polly/LinkAllPasses.h

diff --git a/include/polly/LinkAllPasses.h b/include/polly/LinkAllPasses.h
index a2f8f33..d2e10d1 100644
--- a/include/polly/LinkAllPasses.h
+++ b/include/polly/LinkAllPasses.h
@@ -18,7 +18,7 @@
 #include "polly/Support/DumpFunctionPass.h"
 #include "polly/Support/DumpModulePass.h"
 #include "llvm/ADT/StringRef.h"
-#include <cstdlib>
+#include "llvm/Support/AlwaysTrue.h"
 
 namespace llvm {
 class Pass;
@@ -72,11 +72,10 @@
 namespace {
 struct PollyForcePassLinking {
   PollyForcePassLinking() {
-    // We must reference the passes in such a way that compilers will not
-    // delete it all as dead code, even with whole program optimization,
-    // yet is effectively a NO-OP. As the compiler isn't smart enough
-    // to know that getenv() never returns -1, this will do the job.
-    if (std::getenv("bar") != (char *)-1)
+    // We must reference the passes in such a way that compilers will not delete
+    // it all as dead code, even with whole program optimization, yet is
+    // effectively a NO-OP.
+    if (llvm::getNonFoldableAlwaysTrue())
       return;
 
     polly::createCodePreparationPass();