Bug 7458: Remove LLVM definitions for timer primitives

* sources/dfmc/llvm-back-end/llvm-primitives-os.dylan
  (primitive-descriptor primitive-start-timer): Removed.
  (primitive-descriptor primitive-stop-timer): Removed.
DO NOT MERGE peg testworks at 8a1112ca
Bug 7440: Add an ins--if convenience macro to emit conditionals

* sources/lib/llvm/llvm-builder.dylan
  (ins--if): New convenience macro for emitting conditional blocks
   as LLVM IR.
  (do-ins--if): Auxiliary emitter function implementing ins--if.

* sources/lib/llvm/llvm-library.dylan
  (module llvm-builder): Export ins--if.

* sources/lib/llvm/tests/specification.dylan
  (module-spec llvm-builder): Add macro-test ins--if-test.

* sources/lib/llvm/tests/llvm-builder-test.dylan
  (macro-test ins--if-test): New test units for ins--if.
Bug 7440: Add support for LLVM module flags

* sources/lib/llvm/llvm-module.dylan
  (llvm-module-add-flag): New function for adding module flags via the
  !llvm.module.flags named metadata item.

* sources/lib/llvm/llvm-library.dylan
  (module llvm): Export llvm-module-add-flag.
Bug 7440: Mark debug metadata for LLVM tools

* sources/lib/llvm/llvm-debug.dylan
  ($llvm-debug-metadata-version): New constant.
  (llvm-make-dbg-compile-unit): When debug information is used, mark
   the module using the "Debug Info Version" module flag.
Bug 7458: Fix instance comparisons against <boolean>

* sources/dfmc/llvm-back-end/llvm-emit-type-check.dylan
  (do-emit-instance-cmp): Fix instance? comparison result for #f, and
   simplify code using ins--if.
Bug 7458: Snapshot