[llvm] fix extern cl::opt definitions for DLL export (#145374)

## Purpose
This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch ensures a few `cl::opt` declarations
are properly annotated with `LLVM_ABI`. The annotations currently have
no meaningful impact on the LLVM build; however, they are a prerequisite
to support an LLVM Windows DLL (shared library) build.

## Background
This effort is tracked in #109483. Additional context is provided in
[this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and documentation for `LLVM_ABI` and related annotations is found in the
LLVM repo
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).

## Overview
- Remove local `extern` declarations of `llvm::PrintPipelinePasses`
because it is already correctly declared with an `LLVM_ABI` annotation
in `llvm\Passes\PassBuilder.h`. Leaving these declarations results in a
gcc compile warning unless they are also annotated with `LLVM_ABI`.
- Similarly, remove local `extern` declarations of
`ProfileSummaryCutoffHot` and `UseContextLessSummary` from
`llvm/tools/llvm-profgen/ProfileGenerator.cpp` since they are declared
with `LLVM_ABI` in `llvm\ProfileData\ProfileCommon.h`.
- Explicitly annotate the extern declaration of `ProfileCorrelate` in
`clang/lib/CodeGen/BackendUtil.cpp` since it is not declared in a
header. The definition of `ProfileCorrelate` in
`llvm\lib\Transforms\Instrumentation\InstrProfiling.cpp` is already
annotated with `LLVM_ABI`.

## Validation
Local builds and tests to validate cross-platform compatibility. This
included llvm, clang, and lldb on the following configurations:

- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang

Author: andrurogerz

clang/lib/CodeGen/BackendUtil.cpp

@@ -43,6 +43,7 @@
 #include "llvm/ProfileData/InstrProfCorrelator.h"
 #include "llvm/Support/BuryPointer.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Program.h"
@@ -96,8 +97,6 @@ using namespace llvm;
 #include "llvm/Support/Extension.def"
 
 namespace llvm {
-extern cl::opt<bool> PrintPipelinePasses;
-
 // Experiment to move sanitizers earlier.
 static cl::opt<bool> ClSanitizeOnOptimizerEarlyEP(
     "sanitizer-early-opt-ep", cl::Optional,
@@ -118,7 +117,8 @@ static cl::opt<PGOOptions::ColdFuncOpt> ClPGOColdFuncAttr(
                clEnumValN(PGOOptions::ColdFuncOpt::OptNone, "optnone",
                           "Mark cold functions with optnone.")));
 
-extern cl::opt<InstrProfCorrelator::ProfCorrelatorKind> ProfileCorrelate;
+LLVM_ABI extern cl::opt<InstrProfCorrelator::ProfCorrelatorKind>
+    ProfileCorrelate;
 } // namespace llvm
 namespace clang {
 extern llvm::cl::opt<bool> ClSanitizeGuardChecks;

flang/lib/Frontend/FrontendActions.cpp

@@ -74,10 +74,6 @@
 #include <memory>
 #include <system_error>
 
-namespace llvm {
-extern cl::opt<bool> PrintPipelinePasses;
-} // namespace llvm
-
 using namespace Fortran::frontend;
 
 constexpr llvm::StringLiteral timingIdParse = "Parse";

llvm/tools/llvm-profgen/ProfileGenerator.cpp

@@ -108,8 +108,6 @@ using namespace llvm;
 using namespace sampleprof;
 
 namespace llvm {
-extern cl::opt<int> ProfileSummaryCutoffHot;
-extern cl::opt<bool> UseContextLessSummary;
 
 namespace sampleprof {