Fix LTCG linker flags for macOS with separate debug info

The linker must not throw away the lto.o file. We now instruct the
linker to create a non-temporary lto.o, dependent on the target name.

In order to do that we introduce a new mkspec variable
QMAKE_LFLAGS_LTCG_SEPARATE_DEBUG_INFO. This variable can contain
single-$ variable references that get evaluated when loading ltcg.prf.

Fixes: QTBUG-72846
Change-Id: I0ea882628d63e5406ba0ee68c7435af597364b0f
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

mkspecs/common/clang-mac.conf

@@ -5,6 +5,7 @@ QMAKE_OBJCXXFLAGS_PRECOMPILE     = -x objective-c++-header -c ${QMAKE_PCH_INPUT}
 QMAKE_OBJCXXFLAGS_USE_PRECOMPILE = $$QMAKE_CFLAGS_USE_PRECOMPILE
 
 QMAKE_XCODE_GCC_VERSION = com.apple.compilers.llvm.clang.1_0
+QMAKE_LFLAGS_LTCG_SEPARATE_DEBUG_INFO = -Wl,-object_path_lto,${OBJECTS_DIR}/${TARGET}_lto.o
 
 QMAKE_CXXFLAGS += -stdlib=libc++
 QMAKE_LFLAGS   += -stdlib=libc++

mkspecs/features/ltcg.prf

@@ -1,6 +1,12 @@
 static:no-static-ltcg {
     # Static library but no-static-ltcg enabled: skip LTCG
 } else: CONFIG(release, debug|release) {
+    separate_debug_info {
+        # Evaluate single-$ variable references that have no valid value at mkspec loading time
+        QMAKE_LFLAGS_LTCG_SEPARATE_DEBUG_INFO ~= s/\\$\\{/\$\$\{/
+        eval(QMAKE_LFLAGS_LTCG += $$QMAKE_LFLAGS_LTCG_SEPARATE_DEBUG_INFO)
+    }
+
     # We need fat object files when creating static libraries on some platforms
     # so the linker will know to load a particular object from the library
     # in the first place. On others, we have special ar and nm to create the symbol