[driver] Explicitly specify `-fbuild-session-timestamp` in seconds.

[vsapsai]

Summary

Representation of the file's last modification time depends on the file
system and isn't guaranteed to be in seconds. Cast to seconds explicitly
and tighten the test case to check the magnitude of the calculated
value, so we can catch passing milliseconds or nanoseconds.

rdar://83915615

[vsapsai]

Ping. Failed tests seem to be unrelated (don't know how this change would cause "flangOmpReport.so: cannot open shared object file: No such file or directory"). If you think it would be better, can rebase and trigger new builds.

[vsapsai]

Thanks for the review!

clang/lib/Driver/ToolChains/Clang.cpp

static void RenderModulesOptions(Compilation &C, const Driver &D,
   if (Arg *A = Args.getLastArg(options::OPT_fbuild_session_file)) {
     if (Args.hasArg(options::OPT_fbuild_session_timestamp))
       D.Diag(diag::err_drv_argument_not_allowed_with)
           << A->getAsString(Args) << "-fbuild-session-timestamp";
 
     llvm::sys::fs::file_status Status;
     if (llvm::sys::fs::status(A->getValue(), Status))
       D.Diag(diag::err_drv_no_such_file) << A->getValue();
-    CmdArgs.push_back(
-        Args.MakeArgString("-fbuild-session-timestamp=" +
-                           Twine((uint64_t)Status.getLastModificationTime()
-                                     .time_since_epoch()
-                                     .count())));
+    CmdArgs.push_back(Args.MakeArgString(
+        "-fbuild-session-timestamp=" +
+        Twine((uint64_t)std::chrono::duration_cast<std::chrono::seconds>(
+                  Status.getLastModificationTime().time_since_epoch())
   }
 
   if (Args.getLastArg(options::OPT_fmodules_validate_once_per_build_session)) {
     if (!Args.getLastArg(options::OPT_fbuild_session_timestamp,
                          options::OPT_fbuild_session_file))
       D.Diag(diag::err_drv_modules_validate_once_requires_timestamp);
 
     Args.AddLastArg(CmdArgs,

clang/test/Driver/modules.m

 // RUN: %clang -fmodules -fno-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MODULES %s
 // CHECK-NO-MODULES-NOT: -fmodules
 
 // RUN: %clang -fmodules -fno-modules -fmodules -### %s 2>&1 | FileCheck -check-prefix=CHECK-HAS-MODULES %s
 // CHECK-HAS-MODULES: -fmodules
 
 // RUN: %clang -fbuild-session-file=doesntexist -### %s 2>&1 | FileCheck -check-prefix=NOFILE %s
 // NOFILE: no such file or directory: 'doesntexist'
 
 // RUN: touch -m -a -t 201008011501 %t.build-session-file
 // RUN: %clang -fbuild-session-file=%t.build-session-file -### %s 2>&1 | FileCheck -check-prefix=TIMESTAMP_ONLY %s
 
 // RUN: %clang -fbuild-session-timestamp=1280703457 -### %s 2>&1 | FileCheck -check-prefix=TIMESTAMP_ONLY %s
-// TIMESTAMP_ONLY: -fbuild-session-timestamp=128
+// TIMESTAMP_ONLY: -fbuild-session-timestamp=128{{([[:digit:]]{7})[^[:digit:]]}}
 
 // RUN: %clang -fbuild-session-file=%t.build-session-file -fbuild-session-timestamp=123 -### %s 2>&1 | FileCheck -check-prefix=CONFLICT %s
 // CONFLICT: error: invalid argument '-fbuild-session-file={{.*}}.build-session-file' not allowed with '-fbuild-session-timestamp'
 
 // RUN: %clang -fbuild-session-timestamp=123 -fmodules-validate-once-per-build-session -### %s 2>&1 | FileCheck -check-prefix=MODULES_VALIDATE_ONCE %s
 // MODULES_VALIDATE_ONCE: -fbuild-session-timestamp=123
 // MODULES_VALIDATE_ONCE: -fmodules-validate-once-per-build-session
 
 // RUN: %clang -fbuild-session-file=%t.build-session-file -fmodules-validate-once-per-build-session -### %s 2>&1 | FileCheck -check-prefix=MODULES_VALIDATE_ONCE_FILE %s
-// MODULES_VALIDATE_ONCE_FILE: -fbuild-session-timestamp=128
+// MODULES_VALIDATE_ONCE_FILE: -fbuild-session-timestamp=128{{([[:digit:]]{7})[^[:digit:]]}}
 // MODULES_VALIDATE_ONCE_FILE: -fmodules-validate-once-per-build-session
 
 // RUN: %clang -fmodules-validate-once-per-build-session -### %s 2>&1 | FileCheck -check-prefix=MODULES_VALIDATE_ONCE_ERR %s
 // MODULES_VALIDATE_ONCE_ERR: option '-fmodules-validate-once-per-build-session' requires '-fbuild-session-timestamp=<seconds since Epoch>' or '-fbuild-session-file=<file>'
 
 // RUN: %clang -### %s 2>&1 | FileCheck -check-prefix=MODULES_VALIDATE_SYSTEM_HEADERS_DEFAULT %s
 // MODULES_VALIDATE_SYSTEM_HEADERS_DEFAULT-NOT: -fmodules-validate-system-headers