[YAML] Add an optional argument `EnumMask` to the `yaml::IO::bitSetCase()`.

Some bit-set fields used in ELF file headers in fact contain two parts.
The first one is a regular bit-field. The second one is an enumeraion.
For example ELF header `e_flags` for MIPS target might contain the
following values:

Bit-set values:

  EF_MIPS_NOREORDER = 0x00000001
  EF_MIPS_PIC       = 0x00000002
  EF_MIPS_CPIC      = 0x00000004
  EF_MIPS_ABI2      = 0x00000020

Enumeration:

  EF_MIPS_ARCH_32   = 0x50000000
  EF_MIPS_ARCH_64   = 0x60000000
  EF_MIPS_ARCH_32R2 = 0x70000000
  EF_MIPS_ARCH_64R2 = 0x80000000

For printing bit-sets we use the `yaml::IO::bitSetCase()`. It does not
support bit-set/enumeration combinations and prints too many flags from
an enumeration part. This patch fixes this problem. New method
`yaml::IO::maskedBitSetCase()` handle "enumeration" part of bitset
defined by provided mask.

Patch reviewed by Nick Kledzik and Sean Silva.

llvm-svn: 209504