[NFC] Adjust SmallVector.h header to workaround XL build compiler issue

In order to prevent further building issues related to the usage of SmallVector
in other compilation unit, this patch adjusts the llvm.h header as a workaround
instead.

Besides, this patch reverts previous workarounds:

1. Revert "[NFC] Use llvm::SmallVector to workaround XL compiler problem on AIX"
This reverts commit 561fb7f6.

2.Revert "[clang][cli] Fix build failure in CompilerInvocation"
This reverts commit 8dc70bdc.

Differential Revision: https://reviews.llvm.org/D98552