Change Comments SmallVector to std::vector in DebugLocStream [NFC]

Change Comments SmallVector to std::vector in DebugLocStream [NFC]

This changes the 32-element SmallVector to a std::vector. When building
a RelWithDebInfo clang-8 binary, the average size of the vector was
~10000, so it does not seem very beneficial or practical to use a small
vector for that.

The DWARFBytes SmallVector grows in the same way as Comments, so perhaps
that also should be changed to a purely dynamically allocated structure,
but that requires some more code changes, so I let that remain as a
SmallVector for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374871 91177308-0d34-0410-b5e6-96231b3b80d8