Initial support for vectorization using MASSV (IBM MASS vector library)

Part 2 (the Clang portion) of D59881.

This patch (first of two patches) enables the vectorizer to recognize the
IBM MASS vector library routines. This patch specifically adds support for
recognizing the -vector-library=MASSV option, and defines mappings from IEEE
standard scalar math functions to generic PowerPC MASS vector counterparts.
For instance, the generic PowerPC MASS vector entry for double-precision
cbrt function is __cbrtd2_massv.

The second patch will further lower the generic PowerPC vector entries to
PowerPC subtarget-specific entries.
For instance, the PowerPC generic entry cbrtd2_massv is lowered to
cbrtd2_P9 for Power9 subtarget.

The overall support for MASS vector library is presented as such in two patches
for ease of review.

Patch by Jeeva Paudel.

Differential revision: https://reviews.llvm.org/D59881

llvm-svn: 362571