[NFC][LLVM] Namespace cleanup in MetadataLoader.cpp

[jurahul]

• Remove forward declaration of llvm::Argument and include Argument.h instead.
• Restrict scope of anonymous namespaces to just class declarations.
• Move local static function out of anonymous namespace.
• Remove a redundant assert when indexing a vector.

[jurahul]

@dwblaikie @kazutakahirata gentle ping.

[kazutakahirata]

LGTM. Thanks!

[llvm-ci]

LLVM Buildbot has detected a new failure on builder ppc64le-mlir-rhel-clang running on ppc64le-mlir-rhel-test while building llvm at step 6 "test-build-check-mlir-build-only-check-mlir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/29495

Here is the relevant piece of the build log for the reference

Step 6 (test-build-check-mlir-build-only-check-mlir) failure: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
...
PASS: MLIR-Unit :: IR/./MLIRIRTests/38/129 (3483 of 3494)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/11/22 (3484 of 3494)
PASS: MLIR-Unit :: Pass/./MLIRPassTests/10/13 (3485 of 3494)
PASS: MLIR :: mlir-tblgen/cpp-class-comments.td (3486 of 3494)
PASS: MLIR-Unit :: IR/./MLIRIRTests/0/129 (3487 of 3494)
PASS: MLIR-Unit :: IR/./MLIRIRTests/39/129 (3488 of 3494)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/12/22 (3489 of 3494)
PASS: MLIR :: mlir-tblgen/llvm-intrinsics.td (3490 of 3494)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/13/22 (3491 of 3494)
PASS: MLIR :: mlir-reduce/dce-test.mlir (3492 of 3494)
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1800.698660