[ScanDependencies] Fix JSON generation under certain circunstances. (#67246)

The code of `ScanDependencies.cpp` was creating invalid JSON since #66031
because in the case of having `extraPcmArgs` and `swiftOverlayDependencies`,
but not `bridgingHeader`, a comma will not be added at the end of
`extraPcmArgs`, creating an invalid JSON file. Additionally that same PR
added a trailing comma at the end of the `swiftOverlayDependencies`, which
valid JSON does not allow, but that bug was removed in #66366.

Both problems are, however, present in the 5.9 branch, because #66936
included #66031, but not #66366.

Besides fixing the problem in `ScanDependencies.cpp` I modified every test
that uses `--scan-dependencies` to pass the produced JSON through
Python's `json.tool` in order to validate proper JSON is produced. In
most cases I was able to pipe the output of the tool into `FileCheck`,
but in some cases the validation is done by itself because the checks
depend on the exact format generated by `--scan-dependencies`. In
a couple of tests I added a call to `FileCheck` that seemed to be
missing.

Without these changes, two tests seems to be generating invalid JSON in
my machine:

- `ScanDependencies/local_cache_consistency.swift` (which outputs `Expecting ',' delimiter: line 525 column 11 (char 22799)`)
- `ScanDependencies/placholder_overlay_deps.swift`

test/ModuleInterface/clang-session-transitive.swift

@@ -5,6 +5,7 @@
 // RUN: %target-build-swift -module-name TestModule -module-link-name TestModule %S/Inputs/TestModule.swift -enable-library-evolution -emit-module-interface -o %t/TestModule.swiftmodule -swift-version 5 -Xfrontend -disable-implicit-concurrency-module-import -Xfrontend -disable-implicit-string-processing-module-import
 
 // RUN: %target-swift-frontend -scan-dependencies %s -o %t/deps.json -I%t -validate-clang-modules-once -clang-build-session-file %t/Build.session -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import
+// RUN: %validate-json %t/deps.json &>/dev/null
 // RUN: %FileCheck %s < %t/deps.json
 
 import TestModule