[clang] Record mainfile name in the Frontend time trace (PR #99866)

[Ilya Biryukov via cfe-commits]

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@@ -209,7 +208,7 @@ constexpr int slow_init_list[] = {1, 1, 2, 3, 5, 8, 13, 21}; // 25th line
   ASSERT_TRUE(compileFromString(Code, "-std=c++20", "test.cc"));
   std::string Json = teardownProfiler();
   ASSERT_EQ(R"(
-Frontend
+Frontend (, test.cc)
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ilya-biryukov wrote:

NIT: a simple check in the test output code is probably a better trade-off than descreased readability because of a weird-looking`(,`.

https://github.com/llvm/llvm-project/pull/99866