[CMake] Properly set CMAKE_BUILD_TYPE to Debug by default

[bruening]

Summary

PR26666: CMAKE_BUILD_TYPE was previously being reset to blank.
By setting it after project() we ensure that it is not blank.

[beanz]

Actually I don't think this is the right solution.

The CMAKE_BUILD_TYPE variable needs to be cached. Setting it can stay where it is as long as the set command caches.
Changing:

set(CMAKE_BUILD_TYPE "Debug")

-to-

set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)")

Should fix the bug.

[bruening]

Agreed, it may be best if it's cached: other projects have hit issues with having to force-cache it for Ninja generators (https://github.com/DynamoRIO/dynamorio/issues/919). For some reason in this other project we're not caching it for non-Ninja but it is not clear why: https://github.com/DynamoRIO/dynamorio/blob/master/CMakeLists.txt#L132

[bruening]

Switch to caching CMAKE_BUILD_TYPE.

[beanz]

LGTM!

Thanks for investigating and fixing this!

[bruening]

I am not yet a committer: could you commit the patch? Thank you.

[beanz]

Committed in r261273.

Thanks,
-Chris

CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 2.8.12.2)
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(STATUS "No build type selected, default to Debug")
-  set(CMAKE_BUILD_TYPE "Debug")
+  set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)")
 endif()
 
 if(POLICY CMP0022)
   cmake_policy(SET CMP0022 NEW) # automatic when 2.8.12 is required
 endif()
 
 if (POLICY CMP0051)
   # CMake 3.1 and higher include generator expressions of the form