Basic block sections for functions with implicit-section-name attribute

Functions can have section names set via #pragma or section attributes,
basic block sections should be correctly named for such functions.

With #pragma, the expectation is that all functions in that file are placed
in the same section in the final binary. Basic block sections should be
correctly named with the unique flag set so that the final binary has all the
basic blocks of the function in that named section. This patch fixes the bug
by calling getExplictSectionGlobal when implicit-section-name attribute is set
to make sure the function's basic blocks get the correct section name.

Differential Revision: https://reviews.llvm.org/D101311