This patch is part of a patch series to add support for FileCheck
numeric expressions. This specific patch adds a new class to hold
pattern matching global state.
The table holding the values of FileCheck variable constitutes some sort
of global state for the matching phase, yet is passed as parameters of
all functions using it. This commit create a new FileCheckPatternContext
class pointed at from FileCheckPattern. While it increases the line
count, it separates local data from global state. Later commits build
on that to add numeric expression global state to that class.