[AST] Remove ASTContext from getThisType (NFC)

Summary:
https://reviews.llvm.org/D54862 removed the usages of `ASTContext&` from
within the `CXXMethodDecl::getThisType` method. Remove the parameter
altogether, as well as all usages of it. This does not result in any
functional change because the parameter was unused since
https://reviews.llvm.org/D54862.

Test Plan: check-clang

Reviewers: akyrtzi, mikael

Reviewed By: mikael

Subscribers: mehdi_amini, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D56509

llvm-svn: 350914
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 04f77a8..56af640 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1064,7 +1064,7 @@
 
   if (CXXMethodDecl *method = dyn_cast<CXXMethodDecl>(DC)) {
     if (method && method->isInstance())
-      ThisTy = method->getThisType(Context);
+      ThisTy = method->getThisType();
   }
 
   if (ThisTy.isNull() && isLambdaCallOperator(CurContext) &&
@@ -3588,7 +3588,7 @@
   if (getSourceManager().isInSystemHeader(PointeeRD->getLocation()))
     return;
 
-  QualType ClassType = dtor->getThisType(Context)->getPointeeType();
+  QualType ClassType = dtor->getThisType()->getPointeeType();
   if (PointeeRD->isAbstract()) {
     // If the class is abstract, we warn by default, because we're
     // sure the code has undefined behavior.