[TableGen] Add support for C style macro expressions in TableGen

[pchaudhuri16]

Example :
TableGen today cannot parse C style macros like below
ifdef !MACRO1 || MACRO2 && MACRO3

As a result, we end up creating new macros like MACRO1_OR_MACRO2 to achieve the same functionality.
Hence supporting C style macros, will be a useful feature for TableGen.

[DavidSpickett]

The current capabilities:
https://llvm.org/docs/TableGen/ProgRef.html#include-files
https://llvm.org/docs/TableGen/ProgRef.html#preprocessing-facilities