[asan][test][win] Make report_after_syminitialize test less brittle

[alvinhochun]

Summary

Instead of relying on one of the two commands failing on MinGW target,
just use the %if conditional substitution feature from lit, which is
much less brittle.

Amends 5888a47914f44ffaf102fcb7afd3500706fe753f

[mstorsjo]

LGTM, thanks, this makes it much nicer indeed!

compiler-rt/test/asan/TestCases/Windows/report_after_syminitialize.cpp

 // This test works on both MSVC and MinGW targets, but they require different
 // build commands. By default we use DWARF debug info on MinGW and dbghelp does
 // not support it, so we need to specify extra flags to get the compiler to
 // generate PDB debug info.
 
-// The first build command is intended for MSVC target, which fails on MinGW.
-// The second build command contains the flags required to get PDB debug info
-// on a MinGW build, which fails on MSVC.
-
-// RUN: %clangxx_asan -O0 %s -o %t \
-// RUN: || %clangxx_asan -gcodeview -gcolumn-info -Wl,--pdb= -O0 %s -o %t -ldbghelp
+// RUN: %clangxx_asan %if target={{.*-windows-gnu}} %{ -gcodeview -gcolumn-info -Wl,--pdb= -ldbghelp %} -O0 %s -o %t
 // RUN: %env_asan_opts=external_symbolizer_path=non-existent\\\\asdf not %run %t 2>&1 | FileCheck %s
 
 #include <windows.h>
 #include <dbghelp.h>
 
 #pragma comment(lib, "dbghelp")
 
 int main() {