[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/CodeGen/CGBlocks.cpp b/clang/lib/CodeGen/CGBlocks.cpp
index 239a805..fa3c3ee 100644
--- a/clang/lib/CodeGen/CGBlocks.cpp
+++ b/clang/lib/CodeGen/CGBlocks.cpp
@@ -551,7 +551,7 @@
   if (block->capturesCXXThis()) {
     assert(CGF && CGF->CurFuncDecl && isa<CXXMethodDecl>(CGF->CurFuncDecl) &&
            "Can't capture 'this' outside a method");
-    QualType thisType = cast<CXXMethodDecl>(CGF->CurFuncDecl)->getThisType(C);
+    QualType thisType = cast<CXXMethodDecl>(CGF->CurFuncDecl)->getThisType();
 
     // Theoretically, this could be in a different address space, so
     // don't assume standard pointer size/align.