Disabling speculative loads under asan.

Summary:
While instrumenting std::string with asan I discovered that speculative load might load data from poisoned region. Disabling all speculative loads for asan-annotated functions.

The test follows the std::string implementation.

Corresponding CL in llvm: http://reviews.llvm.org/D13264
Patch by Mike Aizatsky, the review page for the CL is http://reviews.llvm.org/D13265

Reviewers: aizatsky

Subscribers: kcc, llvm-commits

Differential Revision: http://reviews.llvm.org/D13905

llvm-svn: 250837