ScopDetect: Only track detection failures if actually needed.

String operations resulted by raw_string_ostream in the INVALID macro can lead
to significant compile-time overhead when compiling large size source code.
This is because raw_string_ostream relies on TypeFinder class, whose
compile-time cost increases as the size of the module increases. This patch
targets to ensure that it only track detection failures if actually needed.
In this way, we can avoid expensive string operations in normal execution.

With this patch file, the relative compile-time cost of Polly-detect pass does
not increase even when compiling very large size source code.

Contributed-by: Star Tan <tanmx_star@yeah.net>
llvm-svn: 187102