[CMake] Export compile commands by default (#364)

* [CMake] Export compile commands by default

The compilation database should be useful for many LLVM development tasks, for example, debugging compilation of a specific TU in the test suite. It is fairly conservative, writing a single new file, whose size is ~9MiB for the default buil on macOS. Users which already enable it should not see any difference. Thus I think we should just write it by default. It shouldnt break tests.

Guarding the setting by `if (CMAKE_EXPORT_COMPILE_COMMANDS STREQUAL "")` to enable users to restore old behavior if needed. `STREQUAL` is required because CMake defines this variable to `""` by default.

LLVM already exports the compile commands, I think without even allowing a user override. Here we're even more conservative.

* change 1 to ON

Author: tomershafir

CMakeLists.txt

@@ -330,6 +330,10 @@ if(TEST_SUITE_COLLECT_COMPILE_TIME)
   set(CMAKE_CXX_LINK_EXECUTABLE "${CMAKE_BINARY_DIR}/tools/timeit --summary <TARGET>.link.time ${CMAKE_CXX_LINK_EXECUTABLE}")
 endif()
 
+if (CMAKE_EXPORT_COMPILE_COMMANDS STREQUAL "")
+  set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+endif()
+
 option(TEST_SUITE_BENCHMARKING_ONLY "Only run the benchmarking only subset" OFF)
 
 option(TEST_SUITE_COLLECT_STATS "Collect LLVM statistics" OFF)