Introduces cmake option `LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING`

[mehdi_amini]

Summary

We recently introduced a feature that enforce at link-time that the
LLVM headers used by a clients are matching the ABI setting of the
LLVM library linked to.

However for clients that are using only headers from ADT and promise
they won't call into LLVM, this is forcing to link libSupport. This
new flag is intended to provide a way to configure LLVM with this
promise for such client.

[bob.wilson]

This looks good to me. I tried it out and it works as expected.

[compnerd]

Thanks for doing this so quickly.

llvm/CMakeLists.txt

else()
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" ON)
 endif()
 
 option(LLVM_ENABLE_EXPENSIVE_CHECKS "Enable expensive checks" OFF)
 
 set(LLVM_ABI_BREAKING_CHECKS "WITH_ASSERTS" CACHE STRING
   "Enable abi-breaking checks.  Can be WITH_ASSERTS, FORCE_ON or FORCE_OFF.")
 
+option(LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING
+  "Disable abi-breaking checks mismatch detection at link-tim." OFF)
+
 option(LLVM_FORCE_USE_OLD_HOST_TOOLCHAIN
        "Set to ON to force using an old, unsupported host toolchain." OFF)
 
 option(LLVM_USE_INTEL_JITEVENTS
   "Use Intel JIT API to inform Intel(R) VTune(TM) Amplifier XE 2011 about JIT code"
   OFF)
 
 if( LLVM_USE_INTEL_JITEVENTS )

llvm/include/llvm/Config/abi-breaking.h.cmake

 /* This file controls the C++ ABI break introduced in LLVM public header. */
 
 #ifndef LLVM_ABI_BREAKING_CHECKS_H
 #define LLVM_ABI_BREAKING_CHECKS_H
 
 /* Define to enable checks that alter the LLVM C++ ABI */
 #cmakedefine01 LLVM_ENABLE_ABI_BREAKING_CHECKS
 
+/* Define to disable the link-time checking of mismatch for
+   LLVM_ENABLE_ABI_BREAKING_CHECKS */
+#cmakedefine01 LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING
+#if !LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING
+
 // ABI_BREAKING_CHECKS protection: provides link-time failure when clients build
 // mismatch with LLVM
 #if defined(_MSC_VER)
 // Use pragma with MSVC
 #define LLVM_XSTR(s) LLVM_STR(s)
 #define LLVM_STR(s) #s
 #pragma detect_mismatch("LLVM_ENABLE_ABI_BREAKING_CHECKS", LLVM_XSTR(LLVM_ENABLE_ABI_BREAKING_CHECKS))
 #undef LLVM_XSTR
 #undef LLVM_STR
 #elif defined(_WIN32) || defined(__CYGWIN__) // Win32 w/o #pragma detect_mismatch
 // FIXME: Implement checks without weak.
 #elif defined(__cplusplus)
 namespace llvm {
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
 extern int EnableABIBreakingChecks;
 __attribute__((weak, visibility ("hidden"))) int *VerifyEnableABIBreakingChecks = &EnableABIBreakingChecks;
 #else
 extern int DisableABIBreakingChecks;
 __attribute__((weak, visibility ("hidden"))) int *VerifyDisableABIBreakingChecks = &DisableABIBreakingChecks;
 #endif
 }
 #endif // _MSC_VER
 
+#endif // LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING
+
 #endif