[Support] Use namespace qualifiers in Parallel.cpp. NFC (#189268)
Replace `namespace llvm { namespace parallel { ... } }` blocks with
`using namespace` and qualified definitions per
https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-define-previously-declared-symbols
Also reformat the TaskGroup constructor to avoid clang-format issues
with #if/#endif split across the initializer list.
