[LSan] Fix tests with some libstdc++ implementations.

Summary:
Newer libstdc++ has global pool, which is filled with objects
allocated during libstdc++ initialization, and never released.
Using use_globals=0 in the lit tests results in these objects
being treated as leaks.

Fix this by porting several tests to plain C, and introducing
a simple sanity test case for __lsan::ScopedDisabler.

Reviewers: kcc, ygribov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14798

llvm-svn: 253576