Add support for three more string_view functions

Add support for three more string_view functions

1) starts_with(char)
2) ends_with(char)
3) find_first_of(char, size_t)

Reimplemented trim in terms of the new starts_with and ends_with.

Tested:
New unit tests.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D129618