[DTLTO][TEST] Fix the DTLTO Clang driver tests failing on some build bots

Not all builds name the compiler executable clang. For example, the
Fuchsia buildbots use llvm as their single toolchain executable name, as
they combine everything together in a busybox-style binary. This is
currently causing the new ps5-dtlto.c to fail on such build bots.

Update the Clang driver tests to simply check that a non-empty path is
provided for the --thinlto-remote-compiler argument, rather than
hardcoding the executable name. The cross-project tests will verify that
the path is valid later.

This is the same fix as applied earlier in #148908. However, that fix
left a case in the dtlto.c test that was subsequently reflected into the
new ps5-dtlto.c test where it caused a failure. Why it doesn't cause a
failure in the existing dtlto.c test is a mystery to me - perhaps the
substring "clang" is now included in the path to the busybox-style binary,
or perhaps that test was disabled for affected buildbots and then not
re-enabled? It's clearly a latent issue though so I have also fixed the
dtlto.c test in this patch.

Should fix the buildbot failures caused by: #158041.

Author: bd1976bris

clang/test/Driver/DTLTO/dtlto.c

@@ -35,7 +35,7 @@
 
 // DEFAULT: ld.lld
 // DEFAULT-SAME: "--thinlto-distributor=d.exe"
-// DEFAULT-SAME: "--thinlto-remote-compiler={{.*}}clang{{[^\"]*}}"
+// DEFAULT-SAME: "--thinlto-remote-compiler={{[^"]+}}"
 
 /// Check that nothing is forwarded when the compiler is not in LTO mode, and that
 /// appropriate unused option warnings are issued.

clang/test/Driver/DTLTO/ps5-dtlto.c

@@ -35,7 +35,7 @@
 
 // DEFAULT: prospero-lld
 // DEFAULT-SAME: "--thinlto-distributor=d.exe"
-// DEFAULT-SAME: "--thinlto-remote-compiler={{.*}}clang{{[^\"]*}}"
+// DEFAULT-SAME: "--thinlto-remote-compiler={{[^"]+}}"
 
 /// Check that the arguments are forwarded unconditionally even when the
 /// compiler is not in LTO mode.