[ASan] Linker-initialize static ScopedInErrorReport::current_error_.

Summary:
Static ScopedInErrorReport::current_error_ can be linker initialized to
shave one global ctor call on application startup and be __asan_init-safe.

Global constructors in ASan runtime are bad because __asan_init runs
from preinit_array, before any such constructors.

Issue: https://github.com/google/sanitizers/issues/194

Reviewers: eugenis, morehouse

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D48141

llvm-svn: 334748