[Support] Change MapVector's default template parameter to SmallVector<*, 0>

SmallVector<*, 0> is often a better replacement for std::vector :
both the object size and the code size are smaller.
(SmallMapVector uses SmallVector as well, but it is not common.)

clang size decreases by 0.0226%.
instructions:u decreases 0.037% when compiling a sqlite3 amalgram.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D156016