[compiler-rt] Reinstate removal of CRT choice flags from CMAKE_*_FLAGS* (#67935)

This reverts one part of commit
9f4dfcb, with a modified comment added
about the code.

Ideally, this would only be reinstated temporarily - but given the
situation in vcpkg, it looks likely that they would keep passing the
duplicate options for quite some time. The conflicting CRT choice
usually are benign but only would cause warnings about one option
overriding the other, if passing e.g. "/MDd /MT".

However when vcpkg currently sets these options in CMAKE_*_FLAGS_DEBUG,
it passes the redundant option /D_DEBUG; thus the compiler finally ends
up with e.g. "/D_DEBUG /MDd /MT", which has the effect of defining
_DEBUG while using a release mode CRT, which allegedly breaks the build.

There's a PR up for removing this redundant /D_DEBUG option in vcpkg in
microsoft/vcpkg#34123. With that in place, this
change wouldn't be strictly needed.

(cherry picked from commit 7bc09a4)

Author: mstorsjo

compiler-rt/CMakeLists.txt

@@ -389,6 +389,25 @@ endif()
 if(MSVC)
   # FIXME: In fact, sanitizers should support both /MT and /MD, see PR20214.
   set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded)
+
+  # Remove any /M[DT][d] flags, and strip any definitions of _DEBUG.
+  # Since we're using CMAKE_MSVC_RUNTIME_LIBRARY (CMP0091 set to NEW),
+  # these options shouldn't be included in these flags variables. However,
+  # package managers that don't know which mechanism is used for passing
+  # CRT choice flags might be passing them both ways - which leads to
+  # duplicate CRT choice options. Thus make sure to strip out these flags
+  # from these variables, when we're forcing a CRT choice other than what
+  # the user requested here.
+  foreach(flag_var
+    CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
+    CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
+    CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
+    CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
+    string(REGEX REPLACE "[/-]M[DT]d" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]MD" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]D_DEBUG" "" ${flag_var} "${${flag_var}}")
+  endforeach()
+
   append_list_if(COMPILER_RT_HAS_Oy_FLAG /Oy- SANITIZER_COMMON_CFLAGS)
   append_list_if(COMPILER_RT_HAS_GS_FLAG /GS- SANITIZER_COMMON_CFLAGS)