[CMake] Don't use CMakePushCheckState

[phosek]

Summary

When we call cmake_pop_check_state, we undo any changes to REQUIRED
variables performed by HandleLLVMOptions which is undesirable. Rather
use replacement which is what we've used prior to 8d26760a.

[smeenai]

LGTM

llvm/runtimes/CMakeLists.txt

if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
   set(LLVM_PATH ${LLVM_BUILD_MAIN_SRC_DIR})
 
   if(APPLE)
     set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
   endif()
 
   include(CheckLibraryExists)
   include(CheckCCompilerFlag)
-  include(CMakePushCheckState)
-
-  cmake_push_check_state()
 
   # We don't have libc++ (yet)...
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++ -nostdlib++")
 
   # ...but we need access to libc++ headers for CMake checks to succeed.
   if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -isystem ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include")
   endif()
 
   # Avoid checking whether the compiler is working.
   set(LLVM_COMPILER_CHECKED ON)
 
   # Handle common options used by all runtimes.
   include(AddLLVM)
   include(HandleLLVMOptions)
   include(FindPythonInterp)
 
-  cmake_pop_check_state()
+  # Remove the -nostdlib++ option we've added earlier.
+  string(REPLACE "-nostdlib++" "" CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
 
   # Use libtool instead of ar if you are both on an Apple host, and targeting Apple.
   if(CMAKE_HOST_APPLE AND APPLE)
     include(UseLibtool)
   endif()
 
   # This can be used to detect whether we're in the runtimes build.
   set(RUNTIMES_BUILD ON)