Set LLVM_FORCE_USE_OLD_TOOLCHAIN to disable VS2019 checks

VS2019 version 1920 in now the default and get tested in
llvm/include/llvm/Support/Compiler.h. This patch propagates
LLVM_FORCE_USE_OLD_TOOLCHAIN macro to disable testing for VS2019.

Differential Revision: https://reviews.llvm.org/D114639

Author: omjavaid

llvm/include/llvm/Config/llvm-config.h.cmake

@@ -109,4 +109,7 @@
 /* Define if building LLVM with BUILD_SHARED_LIBS */
 #cmakedefine LLVM_BUILD_SHARED_LIBS
 
+/* Define if building LLVM with LLVM_FORCE_USE_OLD_TOOLCHAIN_LIBS */
+#cmakedefine LLVM_FORCE_USE_OLD_TOOLCHAIN $(LLVM_FORCE_USE_OLD_TOOLCHAIN)
+
 #endif

llvm/include/llvm/Support/Compiler.h

@@ -90,9 +90,11 @@
 #define LLVM_MSC_PREREQ(version) (_MSC_VER >= (version))
 
 // We require at least VS 2019.
+#if !defined(LLVM_FORCE_USE_OLD_TOOLCHAIN)
 #if !LLVM_MSC_PREREQ(1920)
 #error LLVM requires at least VS 2019.
 #endif
+#endif
 
 #else
 #define LLVM_MSC_PREREQ(version) 0