[compiler-rt] Relax CHECK line in reaches_function.c

In my build environment, the output is a relative path:
```
test/dfsan/reaches_function.c:34 add.dfsan
test/dfsan/reaches_function.c:56 main
```

This fixes check-dfsan in my configuration:
```
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -G Ninja
-DCMAKE_C_COMPILER=$HOME/output/upstream-llvm/bin/clang
-DCMAKE_CXX_COMPILER=$HOME/output/upstream-llvm/bin/clang++
-DCOMPILER_RT_INCLUDE_TESTS=ON
-DLLVM_EXTERNAL_LIT=$HOME/build/upstream-llvm-project-build/bin/llvm-lit
-DLLVM_CMAKE_DIR=$HOME/output/upstream-llvm
-DCOMPILER_RT_DEBUG=OFF
-S $HOME/src/upstream-llvm-project/compiler-rt
-B $HOME/src/upstream-llvm-project/compiler-rt/cmake-build-all-sanitizers
```

Reviewers: browneee, vitalybuka

Reviewed By: vitalybuka

Pull Request: #83089

Author: arichardson

compiler-rt/test/dfsan/reaches_function.c

@@ -32,11 +32,11 @@ void my_dfsan_reaches_function_callback(dfsan_label label, dfsan_origin origin,
 __attribute__((noinline)) uint64_t add(uint64_t *a, uint64_t *b) {
 
   return *a + *b;
-  // CHECK: {{.*}}compiler-rt/test/dfsan/reaches_function.c:[[# @LINE - 1]] add.dfsan
+  // CHECK: reaches_function.c:[[# @LINE - 1]] add.dfsan
   // CHECK-ORIGIN-TRACKING: Origin value: 0x10000002, Taint value was stored to memory at
-  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in add.dfsan {{.*}}compiler-rt/test/dfsan/reaches_function.c:[[# @LINE - 3]]:{{.*}}
+  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in add.dfsan {{.*}}reaches_function.c:[[# @LINE - 3]]:{{.*}}
   // CHECK-ORIGIN-TRACKING: Origin value: 0x1, Taint value was created at
-  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in main {{.*}}compiler-rt/test/dfsan/reaches_function.c:{{.*}}
+  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in main {{.*}}reaches_function.c:{{.*}}
 }
 
 extern void my_dfsan_reaches_function_callback(dfsan_label label,
@@ -54,11 +54,11 @@ int main(int argc, char *argv[]) {
 
   dfsan_set_label(8, &a, sizeof(a));
   uint64_t c = add(&a, &b);
-  // CHECK: {{.*}}compiler-rt/test/dfsan/reaches_function.c:[[# @LINE - 1]] main
+  // CHECK: reaches_function.c:[[# @LINE - 1]] main
   // CHECK-ORIGIN-TRACKING: Origin value: 0x10000002, Taint value was stored to memory at
-  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in add.dfsan {{.*}}compiler-rt/test/dfsan/reaches_function.c:{{.*}}
+  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in add.dfsan {{.*}}reaches_function.c:{{.*}}
   // CHECK-ORIGIN-TRACKING: Origin value: 0x1, Taint value was created at
-  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in main {{.*}}compiler-rt/test/dfsan/reaches_function.c:[[# @LINE - 6]]:{{.*}}
+  // CHECK-ORIGIN-TRACKING: #0 {{.*}} in main {{.*}}reaches_function.c:[[# @LINE - 6]]:{{.*}}
   return c;
 }