After this change, `check-asan-dynamic` should pass on x86_64 MinGW
target, using the llvm-mingw toolchain.
The following is a list of issues fixed:
* `asan_str_test.cpp`: Exclude unintercepted functions on MinGW.
* `asan_test.cpp`: Work around regex limitation of gtest on Windows,
which only affects MinGW target because `long double` has different
size to `double`.
* `TestCases/Windows/report_after_syminitialize.cpp`: Added build
command specifically for MinGW.
* Other tests: Mark XFAIL for various reasons. Some of them need
further investigation.
Differential Revision: https://reviews.llvm.org/D147059
