[llvm-profdata] Fixed various issue with Sample Profile Reader

Fixed various undefind behaviors with current Sample Profile Reader when reading unusual input. Furthermore, add the following rule on allowing multiple name table sections (current Reader has conflicted code handling such case):

When a new name table section is read (in the order sections are read), the names in the previous name table are cleared. Any subsequent sections referring to function names will index into the most recent read name table.

Also changed name table index to uint64_t to be consistent since there's a mix of using uint32_t and uint64_t.

Reviewed By: snehasish, huangjd

Differential Revision: https://reviews.llvm.org/D146182