[NFC] Modernize enums TypeSpecifierWidth, TypeSpecifierSign &...

[NFC] Modernize enums TypeSpecifierWidth, TypeSpecifierSign & TypeSpecifierType into scoped enums with underlying types.
  - Since these enums are used as bit-fields - for the bit-fields to be interpreted as unsigned, the underlying type must be specified as unsigned.

Previous failed attempt - wherein I did not specify an underlying type - was the sum of:
https://reviews.llvm.org/rC321614
https://reviews.llvm.org/rC321615

llvm-svn: 321622