diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -389,17 +389,6 @@
 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.
-  # TODO: We probably could remove this altogether.
-  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)
 
diff --git a/llvm/cmake/modules/ChooseMSVCCRT.cmake b/llvm/cmake/modules/ChooseMSVCCRT.cmake
--- a/llvm/cmake/modules/ChooseMSVCCRT.cmake
+++ b/llvm/cmake/modules/ChooseMSVCCRT.cmake
@@ -4,52 +4,14 @@
 #
 # The macro is invoked at the end of the file.
 #
-# CMake already sets CRT flags in the CMAKE_CXX_FLAGS_* and
-# CMAKE_C_FLAGS_* variables by default. To let the user
-# override that for each build type:
-# 1. Detect which CRT is already selected, and reflect this in
-# LLVM_USE_CRT_* so the user can have a better idea of what
-# changes they're making.
-# 2. Replace the flags in both variables with the new flag via a regex.
-# 3. set() the variables back into the cache so the changes
-# are user-visible.
-
-### Helper macros: ###
-macro(make_crt_regex regex crts)
-  set(${regex} "")
-  foreach(crt ${${crts}})
-    # Trying to match the beginning or end of the string with stuff
-    # like [ ^]+ didn't work, so use a bunch of parentheses instead.
-    set(${regex} "${${regex}}|(^| +)/${crt}($| +)")
-  endforeach(crt)
-  string(REGEX REPLACE "^\\|" "" ${regex} "${${regex}}")
-endmacro(make_crt_regex)
-
-macro(get_current_crt crt_current regex flagsvar)
-  # Find the selected-by-CMake CRT for each build type, if any.
-  # Strip off the leading slash and any whitespace.
-  string(REGEX MATCH "${${regex}}" ${crt_current} "${${flagsvar}}")
-  string(REPLACE "/" " " ${crt_current} "${${crt_current}}")
-  string(STRIP "${${crt_current}}" ${crt_current})
-endmacro(get_current_crt)
-
-# Replaces or adds a flag to a variable.
-# Expects 'flag' to be padded with spaces.
-macro(set_flag_in_var flagsvar regex flag)
-  string(REGEX MATCH "${${regex}}" current_flag "${${flagsvar}}")
-  if("${current_flag}" STREQUAL "")
-    set(${flagsvar} "${${flagsvar}}${${flag}}")
-  else()
-    string(REGEX REPLACE "${${regex}}" "${${flag}}" ${flagsvar} "${${flagsvar}}")
-  endif()
-  string(STRIP "${${flagsvar}}" ${flagsvar})
-  # Make sure this change gets reflected in the cache/gui.
-  # CMake requires the docstring parameter whenever set() touches the cache,
-  # so get the existing docstring and re-use that.
-  get_property(flagsvar_docs CACHE ${flagsvar} PROPERTY HELPSTRING)
-  set(${flagsvar} "${${flagsvar}}" CACHE STRING "${flagsvar_docs}" FORCE)
-endmacro(set_flag_in_var)
-
+# This mechanism is deprecated, but kept for transitioning users.
+#
+# This reads the LLVM_USE_CRT_<CONFIG> options and sets
+# CMAKE_MSVC_RUNTIME_LIBRARY accordingly. The previous mechanism allowed
+# setting different choices for different build configurations (for
+# multi-config generators), but translating multiple differing choices to
+# the corresponding CMAKE_MSVC_RUNTIME_LIBRARY generator expression isn't
+# supported by this transitional helper.
 
 macro(choose_msvc_crt MSVC_CRT)
   if(LLVM_USE_CRT)
@@ -58,8 +20,6 @@
 variables (LLVM_USE_CRT_DEBUG, etc) instead.")
   endif()
 
-  make_crt_regex(MSVC_CRT_REGEX ${MSVC_CRT})
-
   foreach(build_type ${CMAKE_CONFIGURATION_TYPES} ${CMAKE_BUILD_TYPE})
     string(TOUPPER "${build_type}" build)
     if (NOT "${LLVM_USE_CRT_${build}}" STREQUAL "")
@@ -83,12 +43,6 @@
       message(STATUS "Using VC++ CRT: ${CMAKE_MSVC_RUNTIME_LIBRARY}")
       set(runtime_library_set 1)
     endif()
-    foreach(lang C CXX)
-      # Clear any potentially manually set options from these variables.
-      # Kept as temporary backwards compat (unsure if necessary).
-      # TODO: We probably should remove it.
-      set_flag_in_var(CMAKE_${lang}_FLAGS_${build} MSVC_CRT_REGEX "")
-    endforeach(lang)
   endforeach(build_type)
 endmacro(choose_msvc_crt MSVC_CRT)