[docs] Document LLVM_EXTERNALIZE_DEBUGINFO CMake option

Add `LLVM_EXTERNALIZE_DEBUGINFO` to CMake.rst. This should help make dSYM
generation more discoverable.

Differential Revision: https://reviews.llvm.org/D87591

Author: kastiglione

llvm/docs/CMake.rst

@@ -461,6 +461,10 @@ LLVM-specific variables
 **LLVM_PARALLEL_LINK_JOBS**:STRING
   Define the maximum number of concurrent link jobs.
 
+**LLVM_EXTERNALIZE_DEBUGINFO**:BOOL
+  Generate dSYM files and strip executables and libraries (Darwin Only).
+  Defaults to OFF.
+
 **LLVM_USE_CRT_{target}**:STRING
   On Windows, tells which version of the C runtime library (CRT) should be used.
   For example, -DLLVM_USE_CRT_RELEASE=MT would statically link the CRT into the