[sanitizer] Add dynamic_runtime_thunk for different sanitizers.

In Windows, when the sanitizer is implemented as a shared library (DLL), we need
an auxiliary static library dynamic_runtime_thunk that will be linked to the
main executable and dlls.

In the sanitizer DLL, we are exposing weak functions with WIN_WEAK_EXPORT_DEF(),
which exports the default implementation with __dll suffix. For example: for
sanitizer coverage, the default implementation of __sanitizer_cov_trace_cmp is
exported as: __sanitizer_cov_trace_cmp__dll.

In the dynamic_runtime_thunk static library, we include weak aliases to the
imported implementation from the dll, using the macro WIN_WEAK_IMPORT_DEF().

By default, all users's programs that include calls to weak functions like
__sanitizer_cov_trace_cmp, will be redirected to the implementation in the dll,
when linking to dynamic_runtime_thunk.

After this diff, we are able to compile code with sanitizer coverage
instrumentation on Windows. When the instrumented object files are linked with
clang-rt_asan_dynamic_runtime_thunk-arch.lib all the weak symbols will be
resolved to the implementation imported from asan dll.

All the files sanitizer_dynamic_runtime_thunk.cc are independent, so we do not
need to include a specific list of sanitizers.
Now, we compile: [asan|ubsan|sanitizer_coverage]_win_dynamic_runtime_thunk.cc
and sanitizer_win_dynamic_runtime_thunk.cc to generate
asan_dynamic_runtime_thunk.lib, because we include asan, ubsan and sanitizer
coverage in the address sanitizer library.

Differential Revision: https://reviews.llvm.org/D29158

llvm-svn: 293953

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