[cmake] Add option to build LLVM using the experimental new pass manager

[phosek]

Have you tried running all tests with LLVM_USE_NEWPM=ON? I tried doing this a while ago and I saw various test failures in Clang.

[phosek]

Tested locally and it seems to be working fine, LGTM.

cmake/modules/HandleLLVMOptions.cmake

 
 # Provide public options to globally control RTTI and EH
 option(LLVM_ENABLE_EH "Enable Exception handling" OFF)
 option(LLVM_ENABLE_RTTI "Enable run time type information" OFF)
 if(LLVM_ENABLE_EH AND NOT LLVM_ENABLE_RTTI)
   message(FATAL_ERROR "Exception handling requires RTTI. You must set LLVM_ENABLE_RTTI to ON")
 endif()
 
+option(LLVM_USE_NEWPM "Build LLVM using the experimental new pass manager" Off)
+mark_as_advanced(LLVM_USE_NEWPM)
+if (LLVM_USE_NEWPM)
+  append("-fexperimental-new-pass-manager"
+    CMAKE_CXX_FLAGS
+    CMAKE_C_FLAGS
+    CMAKE_EXE_LINKER_FLAGS
+    CMAKE_SHARED_LINKER_FLAGS)
+endif()
+
 option(LLVM_ENABLE_IR_PGO "Build LLVM and tools with IR PGO instrumentation (deprecated)" Off)
 mark_as_advanced(LLVM_ENABLE_IR_PGO)
 
 set(LLVM_BUILD_INSTRUMENTED OFF CACHE STRING "Build LLVM and tools with PGO instrumentation. May be specified as IR or Frontend")
 mark_as_advanced(LLVM_BUILD_INSTRUMENTED)
 string(TOUPPER "${LLVM_BUILD_INSTRUMENTED}" uppercase_LLVM_BUILD_INSTRUMENTED)
 
 if (LLVM_BUILD_INSTRUMENTED)

docs/CMake.rst

**LLVM_CCACHE_BUILD**:BOOL
   options, which are passed to the CCACHE_MAXSIZE and CCACHE_DIR environment
   variables, respectively.
 
 **LLVM_FORCE_USE_OLD_TOOLCHAIN**:BOOL
   If enabled, the compiler and standard library versions won't be checked. LLVM
   may not compile at all, or might fail at runtime due to known bugs in these
   toolchains.
 
+**LLVM_USE_NEWPM**:BOOL
+  If enabled, use the experimental new pass manager.
+
 CMake Caches
 ============
 
 Recently LLVM and Clang have been adding some more complicated build system
 features. Utilizing these new features often involves a complicated chain of
 CMake variables passed on the command line. Clang provides a collection of CMake
 cache scripts to make these features more approachable.