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

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.

Reviewers: zturner, pcc, thakis

Subscribers: mgorny, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D51952

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342002 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/CMakeLists.txt

@@ -38,6 +38,12 @@ elseif( CMAKE_HOST_UNIX )
   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
@@ -164,7 +170,7 @@ add_llvm_library(LLVMSupport
   ${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}")