[PDB] Enable parallel ghash type merging by default

Ghashing is probably going to be faster in most cases, even without
precomputed ghashes in object files.

Here is my table of results linking clang.pdb:

-------------------------------
| threads | GHASH   | NOGHASH |
-------------------------------
|  j1     | 51.031s | 25.141s |
|  j2     | 31.079s | 22.109s |
|  j4     | 18.609s | 23.156s |
|  j8     | 11.938s | 21.984s |
| j28     |  8.375s | 18.391s |
-------------------------------

This shows that ghashing is faster if at least four cores are available.
This may make the linker slower if most cores are busy in the middle of
a build, but in that case, the linker probably isn't on the critical
path of the build. Incremental build performance is arguably more
important than highly contended batch build link performance.

The -time output indicates that ghash computation is the dominant
factor:

    Input File Reading:             924 ms (  1.8%)
    GC:                             689 ms (  1.3%)
    ICF:                            527 ms (  1.0%)
    Code Layout:                    414 ms (  0.8%)
    Commit Output File:              24 ms (  0.0%)
    PDB Emission (Cumulative):    49938 ms ( 94.8%)
      Add Objects:                46783 ms ( 88.8%)
        Global Type Hashing:      38983 ms ( 74.0%)
        GHash Type Merging:        5640 ms ( 10.7%)
        Symbol Merging:            2154 ms (  4.1%)
      Publics Stream Layout:        188 ms (  0.4%)
      TPI Stream Layout:             18 ms (  0.0%)
      Commit to Disk:              2818 ms (  5.4%)
  --------------------------------------------------
  Total Link Time:                52669 ms (100.0%)

We can speed that up with a faster content hash (not SHA1).

Differential Revision: https://reviews.llvm.org/D102888