[Clang] Change AnonStructIds in MangleContext to per-function based

Clang is generating different mangled names for the same lambda
function in slightly changed builds (like with non-related
source/Macro change). This is due to the fact that clang uses a
cross-translation-unit sequential string "$_<n>" in lambda's
mangled name. Here, "n" is the AnonStructIds field in MangleContext.

Different mangled names for a unchanged function is undesirable:
it makes perf comparison harder, and can cause some unnecessary
profile mismatch in SampleFDO.

This patch makes mangled name for lambda functions more stable
by changing AnonStructIds to a per-function based seq number if the
DeclContext is a function.

Differential Revision: https://reviews.llvm.org/D136397