[PGO] Supporting code for always instrumenting entry block

This patch includes the supporting code that enables always
instrumenting the function entry block by default.

This patch will NOT the default behavior.

It adds a variant bit in the profile version, adds new directives in
text profile format, and changes llvm-profdata tool accordingly.

This patch is a split of D83024 (https://reviews.llvm.org/D83024)
Many test changes from D83024 are also included.

Differential Revision: https://reviews.llvm.org/D84261