[sancov] Define delimiters for sanitizer coverage's binary section on Windows.

On Windows, the symbols "___stop___sancov_guards" and "___start___sancov_guards"
are not defined automatically. So, we need to take a different approach.
We define 3 sections: ".SCOV$A", ".SCOV$M" and ".SCOV$Z".

 Section ".SCOV$A" will only hold a variable ___start___sancov_guard.
 Section ".SCOV$M" will hold the main data.
 Section ".SCOV$Z" will only hold a variable ___stop___sancov_guards.

When linking, they will be merged sorted by the characters after the $, so we
can use the pointers of the variables ___[start|stop]___sancov_guard to know the
actual range of addresses of that section.

___[start|stop]___sancov_guard should be defined only once per module. On
Windows, we have 2 different cases:

+ When considering a shared runtime:
 All the modules, main executable and dlls, are linked to an auxiliary static
 library dynamic_runtime_thunk.lib. Because of that, we include the delimiters
 in `SancovDynamicRuntimeThunk`.

+ When considering a static runtime:
 The main executable in linked to the static runtime library.
 All the dlls are linked to an auxiliary static library dll_thunk.
 Because of that, we include the delimiter to both `SancovDllThunk` and
 `SANITIZER_LIBCDEP_SOURCES` (which is included in the static runtime lib).

Differential Revision: https://reviews.llvm.org/D28435

llvm-svn: 293959