[cmake] Speed up check-llvm 5x by delay loading shell32 and ole32

[rnk]

Summary

Previously, check-llvm on my Windows 10 workstation took about 300s to
run, and it would lock up my mouse. Now, it takes just under 60s.
Previously running the tests only used about 15% of the available CPU
time, now it uses up to 60%.

Shell32.dll and ole32.dll have direct dependencies on user32.dll and
gdi32.dll. These DLLs are mostly used to for Windows GUI functionality,
which most LLVM tools don't need. It seems that these DLLs acquire and
release some system resources on startup and exit, and that requires
acquiring the same highly contended lock discussed in this post:
https://randomascii.wordpress.com/2017/07/09/24-core-cpu-and-i-cant-move-my-mouse/

Most LLVM tools still have a transitive dependency on
SHGetKnownFolderPathW, which is used to look up the user home directory
and local AppData directory. We also use SHFileOperationW to recursively
delete a directory, but only LLDB and clang-doc use this today. At some
point, we might consider removing these last shell32.dll deps, but for
now, I would like to take this free performance.

Many thanks to Bruce Dawson for suggesting this fix. He looked at an ETW
trace of an LLVM test run, noticed these DLLs, and suggested avoiding
them.

[zturner]

Amazeballs!

llvm/lib/Support/CMakeLists.txt

elseif( CMAKE_HOST_UNIX )
   if( UNIX AND NOT (BEOS OR HAIKU) )
     set(system_libs ${system_libs} m)
   endif()
   if( FUCHSIA )
     set(system_libs ${system_libs} zircon)
   endif()
 endif( MSVC OR MINGW )
 
+# Delay load shell32.dll if possible to speed up process startup.
+set (delayload_flags)
+if (MSVC)
+  set (delayload_flags delayimp -delayload:shell32.dll -delayload:ole32.dll)
+endif()
+
 add_llvm_library(LLVMSupport
   AMDGPUMetadata.cpp
   APFloat.cpp
   APInt.cpp
   APSInt.cpp
   ARMBuildAttrs.cpp
   ARMAttributeParser.cpp
   ARMWinEH.cpp
# System
   Watchdog.cpp
 
   ADDITIONAL_HEADER_DIRS
   Unix
   Windows
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/ADT
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Support
   ${Backtrace_INCLUDE_DIRS}
-  LINK_LIBS ${system_libs}
+  LINK_LIBS ${system_libs} ${delayload_flags}
   )
 
 set_property(TARGET LLVMSupport PROPERTY LLVM_SYSTEM_LIBS "${system_libs}")