[clang][test] Fix flaky linker checks in msvc-link.c (#228657)

In clang/test/Driver/msvc-link.c, DEBUG-LINK-NOT: lld and DEBUG-LLD-NOT:
link.exe were placed after the *-SAME: "-debug" checks. Because
FileCheck only scans the input after the preceding positive match for
CHECK-NOT, those negative checks never inspected the linker executable
at the start of the command line and instead scanned the trailing
arguments (such as the temporary object file path inside lit's randomly
named lit-tmp-XXXXXXXX scratch directory), causing spurious failures
whenever the scratch directory name contained "lld".

Moreover, DEBUG-LINK: link.exe" was unanchored on the left and could
match the suffix of "lld-link.exe".

Anchor the positive linker executable regexes in DEBUG-LINK and
DEBUG-LLD to the opening quote or path separator within a single quoted
argument ("{{([^"]*[/\\])?}}...") so link.exe cannot match lld-link.exe,
and remove the redundant trailing *-NOT checks.

Assisted by Gemini.

GitOrigin-RevId: 7f211a4737b1b34b52f19330ffb89533ec74a65c