Remove "parse error" in favor of more descriptive diagnostics.

Remove "parse error" in favor of more descriptive diagnostics.

In a few cases clang emitted a rather content-free diagnostic: 'parse error'.
This change replaces two actual cases (template parameter parsing and K&R
parameter declaration parsing) with more specific diagnostics and removes a
third dead case of this in the BalancedDelimiterTracker (the ctor already
checked the invariant necessary to ensure that the diag::parse_error was never
actually used).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154224 91177308-0d34-0410-b5e6-96231b3b80d8