To enable LTCG, clang-cl.exe need "-flto" instead of "-GL". But if you enabled LTO, the lib tool will have to change from "lib.exe" to "llvm-lib.exe" and the link tool also need to change from "link.exe" to "lld-link.exe"(or "ld.lld.exe"/"ld64.lld.exe"/"lld.exe", they are the same), because msvc linker will see these lib files as corrupted files.

Add the following lines to qtbase\mkspecs\win32-clang-msvc\qmake.conf will solve this problem:

QMAKE_LINK = lld-link
QMAKE_LIB = llvm-lib /NOLOGO
QMAKE_CFLAGS_LTCG = -flto
QMAKE_CXXFLAGS_LTCG = $$QMAKE_CFLAGS_LTCG
QMAKE_LFLAGS_LTCG =

Leave QMAKE_LFLAGS_LTCG empty because "lld-link.exe" does not recognize "-flto" or "-GL".

And one more thing, there's a flood of warning messages when I'm compiling qtbase:

 clang-cl.exe: warning: argument unused during compilation: '-Gm-' [-Wunused-command-line-argument]