[Hexagon] Handling of new HVX flags and target-features

This patch has the following changes
A new flag "-mhvx-length={64B|128B}" is introduced to specify the length of the vector.
Previously we have used "-mhvx-double" for 128 Bytes. This adds the target-feature "+hvx-length{64|128}b"

The "-mhvx" flag must be provided on command line to enable HVX for Hexagon. If no -mhvx-length flag
is specified, a default length is picked from the arch mentioned in this priority order from either -mhvx=vxx
or -mcpu. For v60 and v62 the default length is 64 Byte. For unknown versions, the length is 128 Byte. The 
-mhvx flag adds the target-feature "+hvxv{hvx_version}"

The 64 Byte mode is soon going to be deprecated. A warning is emitted if 64 Byte is enabled. A warning is
still emitted for the default 64 Byte as well. This warning can be suppressed with a -Wno flag.

The "-mhvx-double" and "-mno-hvx-double" flags are deprecated. A warning is emitted if the driver sees
them on commandline. "-mhvx-double" is an alias to "-mhvx-length=128B"

The compilation will error out if -mhvx-length is specified with out an -mhvx/-mhvx= flag

The macro HVX_LENGTH is defined and is set to the length of the vector. 
Eg: #define HVX_LENGTH 64

The macro HVX_ARCH is defined and is set to the version of the HVX. 
Eg: #define HVX_ARCH 62

Differential Revision: https://reviews.llvm.org/D38852

llvm-svn: 316102