[NFC][KeyInstr] Add (LLVM_)EXPERIMENTAL_KEY_INSTRUCTIONS (cmake/)definition

Key Instructions will start development behind a compile time flag to avoid
passing on the increased memory usage to all debug builds. We're working on
improving DILocation memory characteristics simultaneously; once that work
lands we can remove `EXPERIMENTAL_KEY_INSTRUCTIONS`.

This patch doesn't add any code, it's just so we can get the SIE buildbot
building with the new option right away.

Author: OCHyams

llvm/CMakeLists.txt

@@ -545,6 +545,9 @@ set(LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING "DISABLED" CACHE STRING
   "Enhance Debugify's line number coverage tracking; enabling this is ABI-breaking. Can be DISABLED, or COVERAGE.")
 set_property(CACHE LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING PROPERTY STRINGS DISABLED COVERAGE)
 
+option(LLVM_EXPERIMENTAL_KEY_INSTRUCTIONS
+  "Add additional fields to DILocations to support Key Instructions" OFF)
+
 set(WINDOWS_PREFER_FORWARD_SLASH_DEFAULT OFF)
 if (MINGW)
   # Cygwin doesn't identify itself as Windows, and thus gets path::Style::posix

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -206,6 +206,10 @@ else()
   message(FATAL_ERROR "Unknown value for LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING: \"${LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING}\"!")
 endif()
 
+if(LLVM_EXPERIMENTAL_KEY_INSTRUCTIONS)
+  add_compile_definitions(EXPERIMENTAL_KEY_INSTRUCTIONS)
+endif()
+
 if( LLVM_REVERSE_ITERATION )
   set( LLVM_ENABLE_REVERSE_ITERATION 1 )
 endif()