[CMake] Respect LLVM instrumentation variables in AddPureSwift.cmake (#71135)

If one is building Swift against an instrumented LLVM build, when the
functions in AddPureSwift.cmake tries to link Swift code that also links
LLVM instrumented code, the linking will fail because the linker will
not use the profiling runtimes.

The modifications replicate the existing code in LLVM
HandleLLVMOptions.cmake, but adapted to Swift variables where possible.

The necessary arguments are forwarded through `-Xclang-linker` because
the spelling of the Swift driver does not give access to all the options
that Clang provides.

This has been tested with an unified build, but a standalone build could
still pass the `LLVM_*` variables to the Swift build system to make use
of this code.

Author: drodriguez

cmake/modules/AddPureSwift.cmake

@@ -264,6 +264,23 @@ function(add_pure_swift_host_library name)
     )
   endif()
 
+  # This replicates he code existing in LLVM llvm/cmake/modules/HandleLLVMOptions.cmake
+  # The second part of the clause replicates the LINKER_IS_LLD_LINK of the
+  # original.
+  if(LLVM_BUILD_INSTRUMENTED AND NOT (SWIFT_COMPILER_IS_MSVC_LIKE AND SWIFT_USE_LINKER STREQUAL "lld"))
+    string(TOUPPER "${LLVM_BUILD_INSTRUMENTED}" uppercase_LLVM_BUILD_INSTRUMENTED)
+    if(LLVM_ENABLE_IR_PGO OR uppercase_LLVM_BUILD_INSTRUMENTED STREQUAL "IR")
+      target_link_options(${name} PRIVATE
+        "SHELL:-Xclang-linker -fprofile-generate=\"${LLVM_PROFILE_DATA_DIR}\"")
+    elseif(uppercase_LLVM_BUILD_INSTRUMENTED STREQUAL "CSIR")
+      target_link_options(${name} PRIVATE
+        "SHELL:-Xclang-linker -fcs-profile-generate=\"${LLVM_CSPROFILE_DATA_DIR}\"")
+    else()
+      target_link_options(${name} PRIVATE
+        "SHELL:-Xclang-linker -fprofile-instr-generate=\"${LLVM_PROFILE_FILE_PATTERN}\"")
+    endif()
+  endif()
+
   # Export this target.
   set_property(GLOBAL APPEND PROPERTY SWIFT_EXPORTS ${name})
 endfunction()