[tablegen] Emit string literals instead of char arrays

This changes the generated (Instr|Asm|Reg|Regclass)Name tables from this
form:
    extern const char HexagonInstrNameData[] = {
      /* 0 */ 'G', '_', 'F', 'L', 'O', 'G', '1', '0', 0,
      /* 9 */ 'E', 'N', 'D', 'L', 'O', 'O', 'P', '0', 0,
      /* 18 */ 'V', '6', '_', 'v', 'd', 'd', '0', 0,
      /* 26 */ 'P', 'S', '_', 'v', 'd', 'd', '0', 0,
      [...]
    };

...to this:

    extern const char HexagonInstrNameData[] = {
      /* 0 */ "G_FLOG10\0"
      /* 9 */ "ENDLOOP0\0"
      /* 18 */ "V6_vdd0\0"
      /* 26 */ "PS_vdd0\0"
      [...]
    };

This should make debugging and exploration a lot easier for mortals,
while providing a significant compile-time reduction for common compilers.

To avoid issues with low implementation limits, this is disabled by
default for visual studio or when cross-compiling.

To force output one way or the other, pass
`--long-string-literals=<bool>` to `tablegen`

Reviewers: mstorsjo, rnk

Subscribers: llvm-commit

Differential Revision: https://reviews.llvm.org/D73044