[SE] Stop using llvm-config --cxxflags

Summary:
Build configuration was adding $(llvm-config --cxxflags) to the
StreamExecutor CXXFLAGS, but this was causing "-O3" to be passed even
for debug builds, and was making debugging difficult.

The llvm-config call was originally introduced to handle the -fno-rtti
flag because an RTTI StreamExecutor could not link with a no-RTTI LLVM.
This patch converts to using LLVM_ENABLE_RTTI and only adding the
`-fno-rtti` flag if needed, not all the rest of the LLVM CXXFLAGS.

I have tested this with clang-4.0 and gcc-4.8 on Ubuntu. Some work will
probably have to be done to support MSVC.

Reviewers: jlebar

Subscribers: beanz, jprice, parallel_libs-commits, mgorny

Differential Revision: https://reviews.llvm.org/D24474

llvm-svn: 281347

parallel-libs/streamexecutor/CMakeLists.txt

@@ -26,17 +26,10 @@ if(STREAM_EXECUTOR_STANDALONE)
     include_directories(${LLVM_INCLUDE_DIRS})
     add_definitions(${LLVM_DEFINITIONS})
 
-    # Get the LLVM cxxflags by using llvm-config.
-    #
-    # This is necessary to get -fno-rtti if LLVM is compiled that way.
-    execute_process(
-        COMMAND
-        "${LLVM_BINARY_DIR}/bin/llvm-config"
-        --cxxflags
-        OUTPUT_VARIABLE
-        LLVM_CXXFLAGS
-        OUTPUT_STRIP_TRAILING_WHITESPACE)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${LLVM_CXXFLAGS}")
+    # If LLVM does not have RTTI, don't use it here either.
+    if (NOT LLVM_ENABLE_RTTI)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-rtti")
+    endif()
 
     set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")