Make LLVM_ENABLE_CRASH_DUMPS set a variable default

That way users can set the default to true, but then disable core dumps
from certain apps that link support.

llvm-svn: 340588
diff --git a/llvm/lib/Support/Process.cpp b/llvm/lib/Support/Process.cpp
index bf5779d..f32355a 100644
--- a/llvm/lib/Support/Process.cpp
+++ b/llvm/lib/Support/Process.cpp
@@ -83,12 +83,11 @@
  { ALLCOLORS("4",""), ALLCOLORS("4","1;") }
 };
 
-// This is set to true when Process::PreventCoreFiles() is called.
-static bool coreFilesPrevented = false;
+// A CMake option controls wheter we emit core dumps by default. An application
+// may disable core dumps by calling Process::PreventCoreFiles().
+static bool coreFilesPrevented = !LLVM_ENABLE_CRASH_DUMPS;
 
-bool Process::AreCoreFilesPrevented() {
-  return LLVM_ENABLE_CRASH_DUMPS ? coreFilesPrevented : false;
-}
+bool Process::AreCoreFilesPrevented() { return coreFilesPrevented; }
 
 // Include the platform-specific parts of this class.
 #ifdef LLVM_ON_UNIX